| ----------------------------------------------------------------------------- |
| This file contains a concatenation of the PCRE man pages, converted to plain |
| text format for ease of searching with a text editor, or for use on systems |
| that do not have a man page processor. The small individual files that give |
| synopses of each function in the library have not been included. Neither has |
| the pcredemo program. There are separate text files for the pcregrep and |
| pcretest commands. |
| ----------------------------------------------------------------------------- |
| |
| |
| PCRE(3) PCRE(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions |
| |
| |
| INTRODUCTION |
| |
| The PCRE library is a set of functions that implement regular expres- |
| sion pattern matching using the same syntax and semantics as Perl, with |
| just a few differences. Some features that appeared in Python and PCRE |
| before they appeared in Perl are also available using the Python syn- |
| tax, there is some support for one or two .NET and Oniguruma syntax |
| items, and there is an option for requesting some minor changes that |
| give better JavaScript compatibility. |
| |
| The current implementation of PCRE corresponds approximately with Perl |
| 5.12, including support for UTF-8 encoded strings and Unicode general |
| category properties. However, UTF-8 and Unicode support has to be |
| explicitly enabled; it is not the default. The Unicode tables corre- |
| spond to Unicode release 6.0.0. |
| |
| In addition to the Perl-compatible matching function, PCRE contains an |
| alternative function that matches the same compiled patterns in a dif- |
| ferent way. In certain circumstances, the alternative function has some |
| advantages. For a discussion of the two matching algorithms, see the |
| pcrematching page. |
| |
| PCRE is written in C and released as a C library. A number of people |
| have written wrappers and interfaces of various kinds. In particular, |
| Google Inc. have provided a comprehensive C++ wrapper. This is now |
| included as part of the PCRE distribution. The pcrecpp page has details |
| of this interface. Other people's contributions can be found in the |
| Contrib directory at the primary FTP site, which is: |
| |
| ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre |
| |
| Details of exactly which Perl regular expression features are and are |
| not supported by PCRE are given in separate documents. See the pcrepat- |
| tern and pcrecompat pages. There is a syntax summary in the pcresyntax |
| page. |
| |
| Some features of PCRE can be included, excluded, or changed when the |
| library is built. The pcre_config() function makes it possible for a |
| client to discover which features are available. The features them- |
| selves are described in the pcrebuild page. Documentation about build- |
| ing PCRE for various operating systems can be found in the README and |
| NON-UNIX-USE files in the source distribution. |
| |
| The library contains a number of undocumented internal functions and |
| data tables that are used by more than one of the exported external |
| functions, but which are not intended for use by external callers. |
| Their names all begin with "_pcre_", which hopefully will not provoke |
| any name clashes. In some environments, it is possible to control which |
| external symbols are exported when a shared library is built, and in |
| these cases the undocumented symbols are not exported. |
| |
| |
| USER DOCUMENTATION |
| |
| The user documentation for PCRE comprises a number of different sec- |
| tions. In the "man" format, each of these is a separate "man page". In |
| the HTML format, each is a separate page, linked from the index page. |
| In the plain text format, all the sections, except the pcredemo sec- |
| tion, are concatenated, for ease of searching. The sections are as fol- |
| lows: |
| |
| pcre this document |
| pcre-config show PCRE installation configuration information |
| pcreapi details of PCRE's native C API |
| pcrebuild options for building PCRE |
| pcrecallout details of the callout feature |
| pcrecompat discussion of Perl compatibility |
| pcrecpp details of the C++ wrapper |
| pcredemo a demonstration C program that uses PCRE |
| pcregrep description of the pcregrep command |
| pcrejit discussion of the just-in-time optimization support |
| pcrelimits details of size and other limits |
| pcrematching discussion of the two matching algorithms |
| pcrepartial details of the partial matching facility |
| pcrepattern syntax and semantics of supported |
| regular expressions |
| pcreperform discussion of performance issues |
| pcreposix the POSIX-compatible C API |
| pcreprecompile details of saving and re-using precompiled patterns |
| pcresample discussion of the pcredemo program |
| pcrestack discussion of stack usage |
| pcresyntax quick syntax reference |
| pcretest description of the pcretest testing command |
| pcreunicode discussion of Unicode and UTF-8 support |
| |
| In addition, in the "man" and HTML formats, there is a short page for |
| each C library function, listing its arguments and results. |
| |
| |
| AUTHOR |
| |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| Putting an actual email address here seems to have been a spam magnet, |
| so I've taken it away. If you want to email me, use my two initials, |
| followed by the two digits 10, at the domain cam.ac.uk. |
| |
| |
| REVISION |
| |
| Last updated: 24 August 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| |
| |
| PCREBUILD(3) PCREBUILD(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions |
| |
| |
| PCRE BUILD-TIME OPTIONS |
| |
| This document describes the optional features of PCRE that can be |
| selected when the library is compiled. It assumes use of the configure |
| script, where the optional features are selected or deselected by pro- |
| viding options to configure before running the make command. However, |
| the same options can be selected in both Unix-like and non-Unix-like |
| environments using the GUI facility of cmake-gui if you are using CMake |
| instead of configure to build PCRE. |
| |
| There is a lot more information about building PCRE in non-Unix-like |
| environments in the file called NON_UNIX_USE, which is part of the PCRE |
| distribution. You should consult this file as well as the README file |
| if you are building in a non-Unix-like environment. |
| |
| The complete list of options for configure (which includes the standard |
| ones such as the selection of the installation directory) can be |
| obtained by running |
| |
| ./configure --help |
| |
| The following sections include descriptions of options whose names |
| begin with --enable or --disable. These settings specify changes to the |
| defaults for the configure command. Because of the way that configure |
| works, --enable and --disable always come in pairs, so the complemen- |
| tary option always exists as well, but as it specifies the default, it |
| is not described. |
| |
| |
| BUILDING SHARED AND STATIC LIBRARIES |
| |
| The PCRE building process uses libtool to build both shared and static |
| Unix libraries by default. You can suppress one of these by adding one |
| of |
| |
| --disable-shared |
| --disable-static |
| |
| to the configure command, as required. |
| |
| |
| C++ SUPPORT |
| |
| By default, the configure script will search for a C++ compiler and C++ |
| header files. If it finds them, it automatically builds the C++ wrapper |
| library for PCRE. You can disable this by adding |
| |
| --disable-cpp |
| |
| to the configure command. |
| |
| |
| UTF-8 SUPPORT |
| |
| To build PCRE with support for UTF-8 Unicode character strings, add |
| |
| --enable-utf8 |
| |
| to the configure command. Of itself, this does not make PCRE treat |
| strings as UTF-8. As well as compiling PCRE with this option, you also |
| have have to set the PCRE_UTF8 option when you call the pcre_compile() |
| or pcre_compile2() functions. |
| |
| If you set --enable-utf8 when compiling in an EBCDIC environment, PCRE |
| expects its input to be either ASCII or UTF-8 (depending on the runtime |
| option). It is not possible to support both EBCDIC and UTF-8 codes in |
| the same version of the library. Consequently, --enable-utf8 and |
| --enable-ebcdic are mutually exclusive. |
| |
| |
| UNICODE CHARACTER PROPERTY SUPPORT |
| |
| UTF-8 support allows PCRE to process character values greater than 255 |
| in the strings that it handles. On its own, however, it does not pro- |
| vide any facilities for accessing the properties of such characters. If |
| you want to be able to use the pattern escapes \P, \p, and \X, which |
| refer to Unicode character properties, you must add |
| |
| --enable-unicode-properties |
| |
| to the configure command. This implies UTF-8 support, even if you have |
| not explicitly requested it. |
| |
| Including Unicode property support adds around 30K of tables to the |
| PCRE library. Only the general category properties such as Lu and Nd |
| are supported. Details are given in the pcrepattern documentation. |
| |
| |
| JUST-IN-TIME COMPILER SUPPORT |
| |
| Just-in-time compiler support is included in the build by specifying |
| |
| --enable-jit |
| |
| This support is available only for certain hardware architectures. If |
| this option is set for an unsupported architecture, a compile time |
| error occurs. See the pcrejit documentation for a discussion of JIT |
| usage. When JIT support is enabled, pcregrep automatically makes use of |
| it, unless you add |
| |
| --disable-pcregrep-jit |
| |
| to the "configure" command. |
| |
| |
| CODE VALUE OF NEWLINE |
| |
| By default, PCRE interprets the linefeed (LF) character as indicating |
| the end of a line. This is the normal newline character on Unix-like |
| systems. You can compile PCRE to use carriage return (CR) instead, by |
| adding |
| |
| --enable-newline-is-cr |
| |
| to the configure command. There is also a --enable-newline-is-lf |
| option, which explicitly specifies linefeed as the newline character. |
| |
| Alternatively, you can specify that line endings are to be indicated by |
| the two character sequence CRLF. If you want this, add |
| |
| --enable-newline-is-crlf |
| |
| to the configure command. There is a fourth option, specified by |
| |
| --enable-newline-is-anycrlf |
| |
| which causes PCRE to recognize any of the three sequences CR, LF, or |
| CRLF as indicating a line ending. Finally, a fifth option, specified by |
| |
| --enable-newline-is-any |
| |
| causes PCRE to recognize any Unicode newline sequence. |
| |
| Whatever line ending convention is selected when PCRE is built can be |
| overridden when the library functions are called. At build time it is |
| conventional to use the standard for your operating system. |
| |
| |
| WHAT \R MATCHES |
| |
| By default, the sequence \R in a pattern matches any Unicode newline |
| sequence, whatever has been selected as the line ending sequence. If |
| you specify |
| |
| --enable-bsr-anycrlf |
| |
| the default is changed so that \R matches only CR, LF, or CRLF. What- |
| ever is selected when PCRE is built can be overridden when the library |
| functions are called. |
| |
| |
| POSIX MALLOC USAGE |
| |
| When PCRE is called through the POSIX interface (see the pcreposix doc- |
| umentation), additional working storage is required for holding the |
| pointers to capturing substrings, because PCRE requires three integers |
| per substring, whereas the POSIX interface provides only two. If the |
| number of expected substrings is small, the wrapper function uses space |
| on the stack, because this is faster than using malloc() for each call. |
| The default threshold above which the stack is no longer used is 10; it |
| can be changed by adding a setting such as |
| |
| --with-posix-malloc-threshold=20 |
| |
| to the configure command. |
| |
| |
| HANDLING VERY LARGE PATTERNS |
| |
| Within a compiled pattern, offset values are used to point from one |
| part to another (for example, from an opening parenthesis to an alter- |
| nation metacharacter). By default, two-byte values are used for these |
| offsets, leading to a maximum size for a compiled pattern of around |
| 64K. This is sufficient to handle all but the most gigantic patterns. |
| Nevertheless, some people do want to process truyl enormous patterns, |
| so it is possible to compile PCRE to use three-byte or four-byte off- |
| sets by adding a setting such as |
| |
| --with-link-size=3 |
| |
| to the configure command. The value given must be 2, 3, or 4. Using |
| longer offsets slows down the operation of PCRE because it has to load |
| additional bytes when handling them. |
| |
| |
| AVOIDING EXCESSIVE STACK USAGE |
| |
| When matching with the pcre_exec() function, PCRE implements backtrack- |
| ing by making recursive calls to an internal function called match(). |
| In environments where the size of the stack is limited, this can se- |
| verely limit PCRE's operation. (The Unix environment does not usually |
| suffer from this problem, but it may sometimes be necessary to increase |
| the maximum stack size. There is a discussion in the pcrestack docu- |
| mentation.) An alternative approach to recursion that uses memory from |
| the heap to remember data, instead of using recursive function calls, |
| has been implemented to work round the problem of limited stack size. |
| If you want to build a version of PCRE that works this way, add |
| |
| --disable-stack-for-recursion |
| |
| to the configure command. With this configuration, PCRE will use the |
| pcre_stack_malloc and pcre_stack_free variables to call memory manage- |
| ment functions. By default these point to malloc() and free(), but you |
| can replace the pointers so that your own functions are used instead. |
| |
| Separate functions are provided rather than using pcre_malloc and |
| pcre_free because the usage is very predictable: the block sizes |
| requested are always the same, and the blocks are always freed in |
| reverse order. A calling program might be able to implement optimized |
| functions that perform better than malloc() and free(). PCRE runs |
| noticeably more slowly when built in this way. This option affects only |
| the pcre_exec() function; it is not relevant for pcre_dfa_exec(). |
| |
| |
| LIMITING PCRE RESOURCE USAGE |
| |
| Internally, PCRE has a function called match(), which it calls repeat- |
| edly (sometimes recursively) when matching a pattern with the |
| pcre_exec() function. By controlling the maximum number of times this |
| function may be called during a single matching operation, a limit can |
| be placed on the resources used by a single call to pcre_exec(). The |
| limit can be changed at run time, as described in the pcreapi documen- |
| tation. The default is 10 million, but this can be changed by adding a |
| setting such as |
| |
| --with-match-limit=500000 |
| |
| to the configure command. This setting has no effect on the |
| pcre_dfa_exec() matching function. |
| |
| In some environments it is desirable to limit the depth of recursive |
| calls of match() more strictly than the total number of calls, in order |
| to restrict the maximum amount of stack (or heap, if --disable-stack- |
| for-recursion is specified) that is used. A second limit controls this; |
| it defaults to the value that is set for --with-match-limit, which |
| imposes no additional constraints. However, you can set a lower limit |
| by adding, for example, |
| |
| --with-match-limit-recursion=10000 |
| |
| to the configure command. This value can also be overridden at run |
| time. |
| |
| |
| CREATING CHARACTER TABLES AT BUILD TIME |
| |
| PCRE uses fixed tables for processing characters whose code values are |
| less than 256. By default, PCRE is built with a set of tables that are |
| distributed in the file pcre_chartables.c.dist. These tables are for |
| ASCII codes only. If you add |
| |
| --enable-rebuild-chartables |
| |
| to the configure command, the distributed tables are no longer used. |
| Instead, a program called dftables is compiled and run. This outputs |
| the source for new set of tables, created in the default locale of your |
| C runtime system. (This method of replacing the tables does not work if |
| you are cross compiling, because dftables is run on the local host. If |
| you need to create alternative tables when cross compiling, you will |
| have to do so "by hand".) |
| |
| |
| USING EBCDIC CODE |
| |
| PCRE assumes by default that it will run in an environment where the |
| character code is ASCII (or Unicode, which is a superset of ASCII). |
| This is the case for most computer operating systems. PCRE can, how- |
| ever, be compiled to run in an EBCDIC environment by adding |
| |
| --enable-ebcdic |
| |
| to the configure command. This setting implies --enable-rebuild-charta- |
| bles. You should only use it if you know that you are in an EBCDIC |
| environment (for example, an IBM mainframe operating system). The |
| --enable-ebcdic option is incompatible with --enable-utf8. |
| |
| |
| PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT |
| |
| By default, pcregrep reads all files as plain text. You can build it so |
| that it recognizes files whose names end in .gz or .bz2, and reads them |
| with libz or libbz2, respectively, by adding one or both of |
| |
| --enable-pcregrep-libz |
| --enable-pcregrep-libbz2 |
| |
| to the configure command. These options naturally require that the rel- |
| evant libraries are installed on your system. Configuration will fail |
| if they are not. |
| |
| |
| PCREGREP BUFFER SIZE |
| |
| pcregrep uses an internal buffer to hold a "window" on the file it is |
| scanning, in order to be able to output "before" and "after" lines when |
| it finds a match. The size of the buffer is controlled by a parameter |
| whose default value is 20K. The buffer itself is three times this size, |
| but because of the way it is used for holding "before" lines, the long- |
| est line that is guaranteed to be processable is the parameter size. |
| You can change the default parameter value by adding, for example, |
| |
| --with-pcregrep-bufsize=50K |
| |
| to the configure command. The caller of pcregrep can, however, override |
| this value by specifying a run-time option. |
| |
| |
| PCRETEST OPTION FOR LIBREADLINE SUPPORT |
| |
| If you add |
| |
| --enable-pcretest-libreadline |
| |
| to the configure command, pcretest is linked with the libreadline |
| library, and when its input is from a terminal, it reads it using the |
| readline() function. This provides line-editing and history facilities. |
| Note that libreadline is GPL-licensed, so if you distribute a binary of |
| pcretest linked in this way, there may be licensing issues. |
| |
| Setting this option causes the -lreadline option to be added to the |
| pcretest build. In many operating environments with a sytem-installed |
| libreadline this is sufficient. However, in some environments (e.g. if |
| an unmodified distribution version of readline is in use), some extra |
| configuration may be necessary. The INSTALL file for libreadline says |
| this: |
| |
| "Readline uses the termcap functions, but does not link with the |
| termcap or curses library itself, allowing applications which link |
| with readline the to choose an appropriate library." |
| |
| If your environment has not been set up so that an appropriate library |
| is automatically included, you may need to add something like |
| |
| LIBS="-ncurses" |
| |
| immediately before the configure command. |
| |
| |
| SEE ALSO |
| |
| pcreapi(3), pcre_config(3). |
| |
| |
| AUTHOR |
| |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| |
| REVISION |
| |
| Last updated: 06 September 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| |
| |
| PCREMATCHING(3) PCREMATCHING(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions |
| |
| |
| PCRE MATCHING ALGORITHMS |
| |
| This document describes the two different algorithms that are available |
| in PCRE for matching a compiled regular expression against a given sub- |
| ject string. The "standard" algorithm is the one provided by the |
| pcre_exec() function. This works in the same was as Perl's matching |
| function, and provides a Perl-compatible matching operation. |
| |
| An alternative algorithm is provided by the pcre_dfa_exec() function; |
| this operates in a different way, and is not Perl-compatible. It has |
| advantages and disadvantages compared with the standard algorithm, and |
| these are described below. |
| |
| When there is only one possible way in which a given subject string can |
| match a pattern, the two algorithms give the same answer. A difference |
| arises, however, when there are multiple possibilities. For example, if |
| the pattern |
| |
| ^<.*> |
| |
| is matched against the string |
| |
| <something> <something else> <something further> |
| |
| there are three possible answers. The standard algorithm finds only one |
| of them, whereas the alternative algorithm finds all three. |
| |
| |
| REGULAR EXPRESSIONS AS TREES |
| |
| The set of strings that are matched by a regular expression can be rep- |
| resented as a tree structure. An unlimited repetition in the pattern |
| makes the tree of infinite size, but it is still a tree. Matching the |
| pattern to a given subject string (from a given starting point) can be |
| thought of as a search of the tree. There are two ways to search a |
| tree: depth-first and breadth-first, and these correspond to the two |
| matching algorithms provided by PCRE. |
| |
| |
| THE STANDARD MATCHING ALGORITHM |
| |
| In the terminology of Jeffrey Friedl's book "Mastering Regular Expres- |
| sions", the standard algorithm is an "NFA algorithm". It conducts a |
| depth-first search of the pattern tree. That is, it proceeds along a |
| single path through the tree, checking that the subject matches what is |
| required. When there is a mismatch, the algorithm tries any alterna- |
| tives at the current point, and if they all fail, it backs up to the |
| previous branch point in the tree, and tries the next alternative |
| branch at that level. This often involves backing up (moving to the |
| left) in the subject string as well. The order in which repetition |
| branches are tried is controlled by the greedy or ungreedy nature of |
| the quantifier. |
| |
| If a leaf node is reached, a matching string has been found, and at |
| that point the algorithm stops. Thus, if there is more than one possi- |
| ble match, this algorithm returns the first one that it finds. Whether |
| this is the shortest, the longest, or some intermediate length depends |
| on the way the greedy and ungreedy repetition quantifiers are specified |
| in the pattern. |
| |
| Because it ends up with a single path through the tree, it is rela- |
| tively straightforward for this algorithm to keep track of the sub- |
| strings that are matched by portions of the pattern in parentheses. |
| This provides support for capturing parentheses and back references. |
| |
| |
| THE ALTERNATIVE MATCHING ALGORITHM |
| |
| This algorithm conducts a breadth-first search of the tree. Starting |
| from the first matching point in the subject, it scans the subject |
| string from left to right, once, character by character, and as it does |
| this, it remembers all the paths through the tree that represent valid |
| matches. In Friedl's terminology, this is a kind of "DFA algorithm", |
| though it is not implemented as a traditional finite state machine (it |
| keeps multiple states active simultaneously). |
| |
| Although the general principle of this matching algorithm is that it |
| scans the subject string only once, without backtracking, there is one |
| exception: when a lookaround assertion is encountered, the characters |
| following or preceding the current point have to be independently |
| inspected. |
| |
| The scan continues until either the end of the subject is reached, or |
| there are no more unterminated paths. At this point, terminated paths |
| represent the different matching possibilities (if there are none, the |
| match has failed). Thus, if there is more than one possible match, |
| this algorithm finds all of them, and in particular, it finds the long- |
| est. The matches are returned in decreasing order of length. There is |
| an option to stop the algorithm after the first match (which is neces- |
| sarily the shortest) is found. |
| |
| Note that all the matches that are found start at the same point in the |
| subject. If the pattern |
| |
| cat(er(pillar)?)? |
| |
| is matched against the string "the caterpillar catchment", the result |
| will be the three strings "caterpillar", "cater", and "cat" that start |
| at the fifth character of the subject. The algorithm does not automati- |
| cally move on to find matches that start at later positions. |
| |
| There are a number of features of PCRE regular expressions that are not |
| supported by the alternative matching algorithm. They are as follows: |
| |
| 1. Because the algorithm finds all possible matches, the greedy or |
| ungreedy nature of repetition quantifiers is not relevant. Greedy and |
| ungreedy quantifiers are treated in exactly the same way. However, pos- |
| sessive quantifiers can make a difference when what follows could also |
| match what is quantified, for example in a pattern like this: |
| |
| ^a++\w! |
| |
| This pattern matches "aaab!" but not "aaa!", which would be matched by |
| a non-possessive quantifier. Similarly, if an atomic group is present, |
| it is matched as if it were a standalone pattern at the current point, |
| and the longest match is then "locked in" for the rest of the overall |
| pattern. |
| |
| 2. When dealing with multiple paths through the tree simultaneously, it |
| is not straightforward to keep track of captured substrings for the |
| different matching possibilities, and PCRE's implementation of this |
| algorithm does not attempt to do this. This means that no captured sub- |
| strings are available. |
| |
| 3. Because no substrings are captured, back references within the pat- |
| tern are not supported, and cause errors if encountered. |
| |
| 4. For the same reason, conditional expressions that use a backrefer- |
| ence as the condition or test for a specific group recursion are not |
| supported. |
| |
| 5. Because many paths through the tree may be active, the \K escape |
| sequence, which resets the start of the match when encountered (but may |
| be on some paths and not on others), is not supported. It causes an |
| error if encountered. |
| |
| 6. Callouts are supported, but the value of the capture_top field is |
| always 1, and the value of the capture_last field is always -1. |
| |
| 7. The \C escape sequence, which (in the standard algorithm) matches a |
| single byte, even in UTF-8 mode, is not supported in UTF-8 mode, |
| because the alternative algorithm moves through the subject string one |
| character at a time, for all active paths through the tree. |
| |
| 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) |
| are not supported. (*FAIL) is supported, and behaves like a failing |
| negative assertion. |
| |
| |
| ADVANTAGES OF THE ALTERNATIVE ALGORITHM |
| |
| Using the alternative matching algorithm provides the following advan- |
| tages: |
| |
| 1. All possible matches (at a single point in the subject) are automat- |
| ically found, and in particular, the longest match is found. To find |
| more than one match using the standard algorithm, you have to do kludgy |
| things with callouts. |
| |
| 2. Because the alternative algorithm scans the subject string just |
| once, and never needs to backtrack, it is possible to pass very long |
| subject strings to the matching function in several pieces, checking |
| for partial matching each time. Although it is possible to do multi- |
| segment matching using the standard algorithm (pcre_exec()), by retain- |
| ing partially matched substrings, it is more complicated. The pcrepar- |
| tial documentation gives details of partial matching and discusses |
| multi-segment matching. |
| |
| |
| DISADVANTAGES OF THE ALTERNATIVE ALGORITHM |
| |
| The alternative algorithm suffers from a number of disadvantages: |
| |
| 1. It is substantially slower than the standard algorithm. This is |
| partly because it has to search for all possible matches, but is also |
| because it is less susceptible to optimization. |
| |
| 2. Capturing parentheses and back references are not supported. |
| |
| 3. Although atomic groups are supported, their use does not provide the |
| performance advantage that it does for the standard algorithm. |
| |
| |
| AUTHOR |
| |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| |
| REVISION |
| |
| Last updated: 19 November 2011 |
| Copyright (c) 1997-2010 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| |
| |
| PCREAPI(3) PCREAPI(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions |
| |
| |
| PCRE NATIVE API BASIC FUNCTIONS |
| |
| #include <pcre.h> |
| |
| pcre *pcre_compile(const char *pattern, int options, |
| const char **errptr, int *erroffset, |
| const unsigned char *tableptr); |
| |
| pcre *pcre_compile2(const char *pattern, int options, |
| int *errorcodeptr, |
| const char **errptr, int *erroffset, |
| const unsigned char *tableptr); |
| |
| pcre_extra *pcre_study(const pcre *code, int options, |
| const char **errptr); |
| |
| void pcre_free_study(pcre_extra *extra); |
| |
| int pcre_exec(const pcre *code, const pcre_extra *extra, |
| const char *subject, int length, int startoffset, |
| int options, int *ovector, int ovecsize); |
| |
| |
| PCRE NATIVE API AUXILIARY FUNCTIONS |
| |
| pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize); |
| |
| void pcre_jit_stack_free(pcre_jit_stack *stack); |
| |
| void pcre_assign_jit_stack(pcre_extra *extra, |
| pcre_jit_callback callback, void *data); |
| |
| int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, |
| const char *subject, int length, int startoffset, |
| int options, int *ovector, int ovecsize, |
| int *workspace, int wscount); |
| |
| int pcre_copy_named_substring(const pcre *code, |
| const char *subject, int *ovector, |
| int stringcount, const char *stringname, |
| char *buffer, int buffersize); |
| |
| int pcre_copy_substring(const char *subject, int *ovector, |
| int stringcount, int stringnumber, char *buffer, |
| int buffersize); |
| |
| int pcre_get_named_substring(const pcre *code, |
| const char *subject, int *ovector, |
| int stringcount, const char *stringname, |
| const char **stringptr); |
| |
| int pcre_get_stringnumber(const pcre *code, |
| const char *name); |
| |
| int pcre_get_stringtable_entries(const pcre *code, |
| const char *name, char **first, char **last); |
| |
| int pcre_get_substring(const char *subject, int *ovector, |
| int stringcount, int stringnumber, |
| const char **stringptr); |
| |
| int pcre_get_substring_list(const char *subject, |
| int *ovector, int stringcount, const char ***listptr); |
| |
| void pcre_free_substring(const char *stringptr); |
| |
| void pcre_free_substring_list(const char **stringptr); |
| |
| const unsigned char *pcre_maketables(void); |
| |
| int pcre_fullinfo(const pcre *code, const pcre_extra *extra, |
| int what, void *where); |
| |
| int pcre_info(const pcre *code, int *optptr, int *firstcharptr); |
| |
| int pcre_refcount(pcre *code, int adjust); |
| |
| int pcre_config(int what, void *where); |
| |
| char *pcre_version(void); |
| |
| |
| PCRE NATIVE API INDIRECTED FUNCTIONS |
| |
| void *(*pcre_malloc)(size_t); |
| |
| void (*pcre_free)(void *); |
| |
| void *(*pcre_stack_malloc)(size_t); |
| |
| void (*pcre_stack_free)(void *); |
| |
| int (*pcre_callout)(pcre_callout_block *); |
| |
| |
| PCRE API OVERVIEW |
| |
| PCRE has its own native API, which is described in this document. There |
| are also some wrapper functions that correspond to the POSIX regular |
| expression API, but they do not give access to all the functionality. |
| They are described in the pcreposix documentation. Both of these APIs |
| define a set of C function calls. A C++ wrapper is also distributed |
| with PCRE. It is documented in the pcrecpp page. |
| |
| The native API C function prototypes are defined in the header file |
| pcre.h, and on Unix systems the library itself is called libpcre. It |
| can normally be accessed by adding -lpcre to the command for linking an |
| application that uses PCRE. The header file defines the macros |
| PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num- |
| bers for the library. Applications can use these to include support |
| for different releases of PCRE. |
| |
| In a Windows environment, if you want to statically link an application |
| program against a non-dll pcre.a file, you must define PCRE_STATIC |
| before including pcre.h or pcrecpp.h, because otherwise the pcre_mal- |
| loc() and pcre_free() exported functions will be declared |
| __declspec(dllimport), with unwanted results. |
| |
| The functions pcre_compile(), pcre_compile2(), pcre_study(), and |
| pcre_exec() are used for compiling and matching regular expressions in |
| a Perl-compatible manner. A sample program that demonstrates the sim- |
| plest way of using them is provided in the file called pcredemo.c in |
| the PCRE source distribution. A listing of this program is given in the |
| pcredemo documentation, and the pcresample documentation describes how |
| to compile and run it. |
| |
| Just-in-time compiler support is an optional feature of PCRE that can |
| be built in appropriate hardware environments. It greatly speeds up the |
| matching performance of many patterns. Simple programs can easily |
| request that it be used if available, by setting an option that is |
| ignored when it is not relevant. More complicated programs might need |
| to make use of the functions pcre_jit_stack_alloc(), |
| pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to control |
| the JIT code's memory usage. These functions are discussed in the |
| pcrejit documentation. |
| |
| A second matching function, pcre_dfa_exec(), which is not Perl-compati- |
| ble, is also provided. This uses a different algorithm for the match- |
| ing. The alternative algorithm finds all possible matches (at a given |
| point in the subject), and scans the subject just once (unless there |
| are lookbehind assertions). However, this algorithm does not return |
| captured substrings. A description of the two matching algorithms and |
| their advantages and disadvantages is given in the pcrematching docu- |
| mentation. |
| |
| In addition to the main compiling and matching functions, there are |
| convenience functions for extracting captured substrings from a subject |
| string that is matched by pcre_exec(). They are: |
| |
| pcre_copy_substring() |
| pcre_copy_named_substring() |
| pcre_get_substring() |
| pcre_get_named_substring() |
| pcre_get_substring_list() |
| pcre_get_stringnumber() |
| pcre_get_stringtable_entries() |
| |
| pcre_free_substring() and pcre_free_substring_list() are also provided, |
| to free the memory used for extracted strings. |
| |
| The function pcre_maketables() is used to build a set of character |
| tables in the current locale for passing to pcre_compile(), |
| pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is |
| provided for specialist use. Most commonly, no special tables are |
| passed, in which case internal tables that are generated when PCRE is |
| built are used. |
| |
| The function pcre_fullinfo() is used to find out information about a |
| compiled pattern; pcre_info() is an obsolete version that returns only |
| some of the available information, but is retained for backwards com- |
| patibility. The function pcre_version() returns a pointer to a string |
| containing the version of PCRE and its date of release. |
| |
| The function pcre_refcount() maintains a reference count in a data |
| block containing a compiled pattern. This is provided for the benefit |
| of object-oriented applications. |
| |
| The global variables pcre_malloc and pcre_free initially contain the |
| entry points of the standard malloc() and free() functions, respec- |
| tively. PCRE calls the memory management functions via these variables, |
| so a calling program can replace them if it wishes to intercept the |
| calls. This should be done before calling any PCRE functions. |
| |
| The global variables pcre_stack_malloc and pcre_stack_free are also |
| indirections to memory management functions. These special functions |
| are used only when PCRE is compiled to use the heap for remembering |
| data, instead of recursive function calls, when running the pcre_exec() |
| function. See the pcrebuild documentation for details of how to do |
| this. It is a non-standard way of building PCRE, for use in environ- |
| ments that have limited stacks. Because of the greater use of memory |
| management, it runs more slowly. Separate functions are provided so |
| that special-purpose external code can be used for this case. When |
| used, these functions are always called in a stack-like manner (last |
| obtained, first freed), and always for memory blocks of the same size. |
| There is a discussion about PCRE's stack usage in the pcrestack docu- |
| mentation. |
| |
| The global variable pcre_callout initially contains NULL. It can be set |
| by the caller to a "callout" function, which PCRE will then call at |
| specified points during a matching operation. Details are given in the |
| pcrecallout documentation. |
| |
| |
| NEWLINES |
| |
| PCRE supports five different conventions for indicating line breaks in |
| strings: a single CR (carriage return) character, a single LF (line- |
| feed) character, the two-character sequence CRLF, any of the three pre- |
| ceding, or any Unicode newline sequence. The Unicode newline sequences |
| are the three just mentioned, plus the single characters VT (vertical |
| tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line |
| separator, U+2028), and PS (paragraph separator, U+2029). |
| |
| Each of the first three conventions is used by at least one operating |
| system as its standard newline sequence. When PCRE is built, a default |
| can be specified. The default default is LF, which is the Unix stan- |
| dard. When PCRE is run, the default can be overridden, either when a |
| pattern is compiled, or when it is matched. |
| |
| At compile time, the newline convention can be specified by the options |
| argument of pcre_compile(), or it can be specified by special text at |
| the start of the pattern itself; this overrides any other settings. See |
| the pcrepattern page for details of the special character sequences. |
| |
| In the PCRE documentation the word "newline" is used to mean "the char- |
| acter or pair of characters that indicate a line break". The choice of |
| newline convention affects the handling of the dot, circumflex, and |
| dollar metacharacters, the handling of #-comments in /x mode, and, when |
| CRLF is a recognized line ending sequence, the match position advance- |
| ment for a non-anchored pattern. There is more detail about this in the |
| section on pcre_exec() options below. |
| |
| The choice of newline convention does not affect the interpretation of |
| the \n or \r escape sequences, nor does it affect what \R matches, |
| which is controlled in a similar way, but by separate options. |
| |
| |
| MULTITHREADING |
| |
| The PCRE functions can be used in multi-threading applications, with |
| the proviso that the memory management functions pointed to by |
| pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the |
| callout function pointed to by pcre_callout, are shared by all threads. |
| |
| The compiled form of a regular expression is not altered during match- |
| ing, so the same compiled pattern can safely be used by several threads |
| at once. |
| |
| If the just-in-time optimization feature is being used, it needs sepa- |
| rate memory stack areas for each thread. See the pcrejit documentation |
| for more details. |
| |
| |
| SAVING PRECOMPILED PATTERNS FOR LATER USE |
| |
| The compiled form of a regular expression can be saved and re-used at a |
| later time, possibly by a different program, and even on a host other |
| than the one on which it was compiled. Details are given in the |
| pcreprecompile documentation. However, compiling a regular expression |
| with one version of PCRE for use with a different version is not guar- |
| anteed to work and may cause crashes. |
| |
| |
| CHECKING BUILD-TIME OPTIONS |
| |
| int pcre_config(int what, void *where); |
| |
| The function pcre_config() makes it possible for a PCRE client to dis- |
| cover which optional features have been compiled into the PCRE library. |
| The pcrebuild documentation has more details about these optional fea- |
| tures. |
| |
| The first argument for pcre_config() is an integer, specifying which |
| information is required; the second argument is a pointer to a variable |
| into which the information is placed. The following information is |
| available: |
| |
| PCRE_CONFIG_UTF8 |
| |
| The output is an integer that is set to one if UTF-8 support is avail- |
| able; otherwise it is set to zero. |
| |
| PCRE_CONFIG_UNICODE_PROPERTIES |
| |
| The output is an integer that is set to one if support for Unicode |
| character properties is available; otherwise it is set to zero. |
| |
| PCRE_CONFIG_JIT |
| |
| The output is an integer that is set to one if support for just-in-time |
| compiling is available; otherwise it is set to zero. |
| |
| PCRE_CONFIG_NEWLINE |
| |
| The output is an integer whose value specifies the default character |
| sequence that is recognized as meaning "newline". The four values that |
| are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF, |
| and -1 for ANY. Though they are derived from ASCII, the same values |
| are returned in EBCDIC environments. The default should normally corre- |
| spond to the standard sequence for your operating system. |
| |
| PCRE_CONFIG_BSR |
| |
| The output is an integer whose value indicates what character sequences |
| the \R escape sequence matches by default. A value of 0 means that \R |
| matches any Unicode line ending sequence; a value of 1 means that \R |
| matches only CR, LF, or CRLF. The default can be overridden when a pat- |
| tern is compiled or matched. |
| |
| PCRE_CONFIG_LINK_SIZE |
| |
| The output is an integer that contains the number of bytes used for |
| internal linkage in compiled regular expressions. The value is 2, 3, or |
| 4. Larger values allow larger regular expressions to be compiled, at |
| the expense of slower matching. The default value of 2 is sufficient |
| for all but the most massive patterns, since it allows the compiled |
| pattern to be up to 64K in size. |
| |
| PCRE_CONFIG_POSIX_MALLOC_THRESHOLD |
| |
| The output is an integer that contains the threshold above which the |
| POSIX interface uses malloc() for output vectors. Further details are |
| given in the pcreposix documentation. |
| |
| PCRE_CONFIG_MATCH_LIMIT |
| |
| The output is a long integer that gives the default limit for the num- |
| ber of internal matching function calls in a pcre_exec() execution. |
| Further details are given with pcre_exec() below. |
| |
| PCRE_CONFIG_MATCH_LIMIT_RECURSION |
| |
| The output is a long integer that gives the default limit for the depth |
| of recursion when calling the internal matching function in a |
| pcre_exec() execution. Further details are given with pcre_exec() |
| below. |
| |
| PCRE_CONFIG_STACKRECURSE |
| |
| The output is an integer that is set to one if internal recursion when |
| running pcre_exec() is implemented by recursive function calls that use |
| the stack to remember their state. This is the usual way that PCRE is |
| compiled. The output is zero if PCRE was compiled to use blocks of data |
| on the heap instead of recursive function calls. In this case, |
| pcre_stack_malloc and pcre_stack_free are called to manage memory |
| blocks on the heap, thus avoiding the use of the stack. |
| |
| |
| COMPILING A PATTERN |
| |
| pcre *pcre_compile(const char *pattern, int options, |
| const char **errptr, int *erroffset, |
| const unsigned char *tableptr); |
| |
| pcre *pcre_compile2(const char *pattern, int options, |
| int *errorcodeptr, |
| const char **errptr, int *erroffset, |
| const unsigned char *tableptr); |
| |
| Either of the functions pcre_compile() or pcre_compile2() can be called |
| to compile a pattern into an internal form. The only difference between |
| the two interfaces is that pcre_compile2() has an additional argument, |
| errorcodeptr, via which a numerical error code can be returned. To |
| avoid too much repetition, we refer just to pcre_compile() below, but |
| the information applies equally to pcre_compile2(). |
| |
| The pattern is a C string terminated by a binary zero, and is passed in |
| the pattern argument. A pointer to a single block of memory that is |
| obtained via pcre_malloc is returned. This contains the compiled code |
| and related data. The pcre type is defined for the returned block; this |
| is a typedef for a structure whose contents are not externally defined. |
| It is up to the caller to free the memory (via pcre_free) when it is no |
| longer required. |
| |
| Although the compiled code of a PCRE regex is relocatable, that is, it |
| does not depend on memory location, the complete pcre data block is not |
| fully relocatable, because it may contain a copy of the tableptr argu- |
| ment, which is an address (see below). |
| |
| The options argument contains various bit settings that affect the com- |
| pilation. It should be zero if no options are required. The available |
| options are described below. Some of them (in particular, those that |
| are compatible with Perl, but some others as well) can also be set and |
| unset from within the pattern (see the detailed description in the |
| pcrepattern documentation). For those options that can be different in |
| different parts of the pattern, the contents of the options argument |
| specifies their settings at the start of compilation and execution. The |
| PCRE_ANCHORED, PCRE_BSR_xxx, PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK, and |
| PCRE_NO_START_OPT options can be set at the time of matching as well as |
| at compile time. |
| |
| If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise, |
| if compilation of a pattern fails, pcre_compile() returns NULL, and |
| sets the variable pointed to by errptr to point to a textual error mes- |
| sage. This is a static string that is part of the library. You must not |
| try to free it. Normally, the offset from the start of the pattern to |
| the byte that was being processed when the error was discovered is |
| placed in the variable pointed to by erroffset, which must not be NULL |
| (if it is, an immediate error is given). However, for an invalid UTF-8 |
| string, the offset is that of the first byte of the failing character. |
| Also, some errors are not detected until checks are carried out when |
| the whole pattern has been scanned; in these cases the offset passed |
| back is the length of the pattern. |
| |
| Note that the offset is in bytes, not characters, even in UTF-8 mode. |
| It may sometimes point into the middle of a UTF-8 character. |
| |
| If pcre_compile2() is used instead of pcre_compile(), and the error- |
| codeptr argument is not NULL, a non-zero error code number is returned |
| via this argument in the event of an error. This is in addition to the |
| textual error message. Error codes and messages are listed below. |
| |
| If the final argument, tableptr, is NULL, PCRE uses a default set of |
| character tables that are built when PCRE is compiled, using the |
| default C locale. Otherwise, tableptr must be an address that is the |
| result of a call to pcre_maketables(). This value is stored with the |
| compiled pattern, and used again by pcre_exec(), unless another table |
| pointer is passed to it. For more discussion, see the section on locale |
| support below. |
| |
| This code fragment shows a typical straightforward call to pcre_com- |
| pile(): |
| |
| pcre *re; |
| const char *error; |
| int erroffset; |
| re = pcre_compile( |
| "^A.*Z", /* the pattern */ |
| 0, /* default options */ |
| &error, /* for error message */ |
| &erroffset, /* for error offset */ |
| NULL); /* use default character tables */ |
| |
| The following names for option bits are defined in the pcre.h header |
| file: |
| |
| PCRE_ANCHORED |
| |
| If this bit is set, the pattern is forced to be "anchored", that is, it |
| is constrained to match only at the first matching point in the string |
| that is being searched (the "subject string"). This effect can also be |
| achieved by appropriate constructs in the pattern itself, which is the |
| only way to do it in Perl. |
| |
| PCRE_AUTO_CALLOUT |
| |
| If this bit is set, pcre_compile() automatically inserts callout items, |
| all with number 255, before each pattern item. For discussion of the |
| callout facility, see the pcrecallout documentation. |
| |
| PCRE_BSR_ANYCRLF |
| PCRE_BSR_UNICODE |
| |
| These options (which are mutually exclusive) control what the \R escape |
| sequence matches. The choice is either to match only CR, LF, or CRLF, |
| or to match any Unicode newline sequence. The default is specified when |
| PCRE is built. It can be overridden from within the pattern, or by set- |
| ting an option when a compiled pattern is matched. |
| |
| PCRE_CASELESS |
| |
| If this bit is set, letters in the pattern match both upper and lower |
| case letters. It is equivalent to Perl's /i option, and it can be |
| changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE |
| always understands the concept of case for characters whose values are |
| less than 128, so caseless matching is always possible. For characters |
| with higher values, the concept of case is supported if PCRE is com- |
| piled with Unicode property support, but not otherwise. If you want to |
| use caseless matching for characters 128 and above, you must ensure |
| that PCRE is compiled with Unicode property support as well as with |
| UTF-8 support. |
| |
| PCRE_DOLLAR_ENDONLY |
| |
| If this bit is set, a dollar metacharacter in the pattern matches only |
| at the end of the subject string. Without this option, a dollar also |
| matches immediately before a newline at the end of the string (but not |
| before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored |
| if PCRE_MULTILINE is set. There is no equivalent to this option in |
| Perl, and no way to set it within a pattern. |
| |
| PCRE_DOTALL |
| |
| If this bit is set, a dot metacharacter in the pattern matches a char- |
| acter of any value, including one that indicates a newline. However, it |
| only ever matches one character, even if newlines are coded as CRLF. |
| Without this option, a dot does not match when the current position is |
| at a newline. This option is equivalent to Perl's /s option, and it can |
| be changed within a pattern by a (?s) option setting. A negative class |
| such as [^a] always matches newline characters, independent of the set- |
| ting of this option. |
| |
| PCRE_DUPNAMES |
| |
| If this bit is set, names used to identify capturing subpatterns need |
| not be unique. This can be helpful for certain types of pattern when it |
| is known that only one instance of the named subpattern can ever be |
| matched. There are more details of named subpatterns below; see also |
| the pcrepattern documentation. |
| |
| PCRE_EXTENDED |
| |
| If this bit is set, whitespace data characters in the pattern are |
| totally ignored except when escaped or inside a character class. White- |
| space does not include the VT character (code 11). In addition, charac- |
| ters between an unescaped # outside a character class and the next new- |
| line, inclusive, are also ignored. This is equivalent to Perl's /x |
| option, and it can be changed within a pattern by a (?x) option set- |
| ting. |
| |
| Which characters are interpreted as newlines is controlled by the |
| options passed to pcre_compile() or by a special sequence at the start |
| of the pattern, as described in the section entitled "Newline conven- |
| tions" in the pcrepattern documentation. Note that the end of this type |
| of comment is a literal newline sequence in the pattern; escape |
| sequences that happen to represent a newline do not count. |
| |
| This option makes it possible to include comments inside complicated |
| patterns. Note, however, that this applies only to data characters. |
| Whitespace characters may never appear within special character |
| sequences in a pattern, for example within the sequence (?( that intro- |
| duces a conditional subpattern. |
| |
| PCRE_EXTRA |
| |
| This option was invented in order to turn on additional functionality |
| of PCRE that is incompatible with Perl, but it is currently of very |
| little use. When set, any backslash in a pattern that is followed by a |
| letter that has no special meaning causes an error, thus reserving |
| these combinations for future expansion. By default, as in Perl, a |
| backslash followed by a letter with no special meaning is treated as a |
| literal. (Perl can, however, be persuaded to give an error for this, by |
| running it with the -w option.) There are at present no other features |
| controlled by this option. It can also be set by a (?X) option setting |
| within a pattern. |
| |
| PCRE_FIRSTLINE |
| |
| If this option is set, an unanchored pattern is required to match |
| before or at the first newline in the subject string, though the |
| matched text may continue over the newline. |
| |
| PCRE_JAVASCRIPT_COMPAT |
| |
| If this option is set, PCRE's behaviour is changed in some ways so that |
| it is compatible with JavaScript rather than Perl. The changes are as |
| follows: |
| |
| (1) A lone closing square bracket in a pattern causes a compile-time |
| error, because this is illegal in JavaScript (by default it is treated |
| as a data character). Thus, the pattern AB]CD becomes illegal when this |
| option is set. |
| |
| (2) At run time, a back reference to an unset subpattern group matches |
| an empty string (by default this causes the current matching alterna- |
| tive to fail). A pattern such as (\1)(a) succeeds when this option is |
| set (assuming it can find an "a" in the subject), whereas it fails by |
| default, for Perl compatibility. |
| |
| (3) \U matches an upper case "U" character; by default \U causes a com- |
| pile time error (Perl uses \U to upper case subsequent characters). |
| |
| (4) \u matches a lower case "u" character unless it is followed by four |
| hexadecimal digits, in which case the hexadecimal number defines the |
| code point to match. By default, \u causes a compile time error (Perl |
| uses it to upper case the following character). |
| |
| (5) \x matches a lower case "x" character unless it is followed by two |
| hexadecimal digits, in which case the hexadecimal number defines the |
| code point to match. By default, as in Perl, a hexadecimal number is |
| always expected after \x, but it may have zero, one, or two digits (so, |
| for example, \xz matches a binary zero character followed by z). |
| |
| PCRE_MULTILINE |
| |
| By default, PCRE treats the subject string as consisting of a single |
| line of characters (even if it actually contains newlines). The "start |
| of line" metacharacter (^) matches only at the start of the string, |
| while the "end of line" metacharacter ($) matches only at the end of |
| the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY |
| is set). This is the same as Perl. |
| |
| When PCRE_MULTILINE it is set, the "start of line" and "end of line" |
| constructs match immediately following or immediately before internal |
| newlines in the subject string, respectively, as well as at the very |
| start and end. This is equivalent to Perl's /m option, and it can be |
| changed within a pattern by a (?m) option setting. If there are no new- |
| lines in a subject string, or no occurrences of ^ or $ in a pattern, |
| setting PCRE_MULTILINE has no effect. |
| |
| PCRE_NEWLINE_CR |
| PCRE_NEWLINE_LF |
| PCRE_NEWLINE_CRLF |
| PCRE_NEWLINE_ANYCRLF |
| PCRE_NEWLINE_ANY |
| |
| These options override the default newline definition that was chosen |
| when PCRE was built. Setting the first or the second specifies that a |
| newline is indicated by a single character (CR or LF, respectively). |
| Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the |
| two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies |
| that any of the three preceding sequences should be recognized. Setting |
| PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be |
| recognized. The Unicode newline sequences are the three just mentioned, |
| plus the single characters VT (vertical tab, U+000B), FF (formfeed, |
| U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS |
| (paragraph separator, U+2029). The last two are recognized only in |
| UTF-8 mode. |
| |
| The newline setting in the options word uses three bits that are |
| treated as a number, giving eight possibilities. Currently only six are |
| used (default plus the five values above). This means that if you set |
| more than one newline option, the combination may or may not be sensi- |
| ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to |
| PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and |
| cause an error. |
| |
| The only time that a line break in a pattern is specially recognized |
| when compiling is when PCRE_EXTENDED is set. CR and LF are whitespace |
| characters, and so are ignored in this mode. Also, an unescaped # out- |
| side a character class indicates a comment that lasts until after the |
| next line break sequence. In other circumstances, line break sequences |
| in patterns are treated as literal data. |
| |
| The newline option that is set at compile time becomes the default that |
| is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden. |
| |
| PCRE_NO_AUTO_CAPTURE |
| |
| If this option is set, it disables the use of numbered capturing paren- |
| theses in the pattern. Any opening parenthesis that is not followed by |
| ? behaves as if it were followed by ?: but named parentheses can still |
| be used for capturing (and they acquire numbers in the usual way). |
| There is no equivalent of this option in Perl. |
| |
| NO_START_OPTIMIZE |
| |
| This is an option that acts at matching time; that is, it is really an |
| option for pcre_exec() or pcre_dfa_exec(). If it is set at compile |
| time, it is remembered with the compiled pattern and assumed at match- |
| ing time. For details see the discussion of PCRE_NO_START_OPTIMIZE |
| below. |
| |
| PCRE_UCP |
| |
| This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W, |
| \w, and some of the POSIX character classes. By default, only ASCII |
| characters are recognized, but if PCRE_UCP is set, Unicode properties |
| are used instead to classify characters. More details are given in the |
| section on generic character types in the pcrepattern page. If you set |
| PCRE_UCP, matching one of the items it affects takes much longer. The |
| option is available only if PCRE has been compiled with Unicode prop- |
| erty support. |
| |
| PCRE_UNGREEDY |
| |
| This option inverts the "greediness" of the quantifiers so that they |
| are not greedy by default, but become greedy if followed by "?". It is |
| not compatible with Perl. It can also be set by a (?U) option setting |
| within the pattern. |
| |
| PCRE_UTF8 |
| |
| This option causes PCRE to regard both the pattern and the subject as |
| strings of UTF-8 characters instead of single-byte character strings. |
| However, it is available only when PCRE is built to include UTF-8 sup- |
| port. If not, the use of this option provokes an error. Details of how |
| this option changes the behaviour of PCRE are given in the pcreunicode |
| page. |
| |
| PCRE_NO_UTF8_CHECK |
| |
| When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is |
| automatically checked. There is a discussion about the validity of |
| UTF-8 strings in the main pcre page. If an invalid UTF-8 sequence of |
| bytes is found, pcre_compile() returns an error. If you already know |
| that your pattern is valid, and you want to skip this check for perfor- |
| mance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is |
| set, the effect of passing an invalid UTF-8 string as a pattern is |
| undefined. It may cause your program to crash. Note that this option |
| can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the |
| UTF-8 validity checking of subject strings. |
| |
| |
| COMPILATION ERROR CODES |
| |
| The following table lists the error codes than may be returned by |
| pcre_compile2(), along with the error messages that may be returned by |
| both compiling functions. As PCRE has developed, some error codes have |
| fallen out of use. To avoid confusion, they have not been re-used. |
| |
| 0 no error |
| 1 \ at end of pattern |
| 2 \c at end of pattern |
| 3 unrecognized character follows \ |
| 4 numbers out of order in {} quantifier |
| 5 number too big in {} quantifier |
| 6 missing terminating ] for character class |
| 7 invalid escape sequence in character class |
| 8 range out of order in character class |
| 9 nothing to repeat |
| 10 [this code is not in use] |
| 11 internal error: unexpected repeat |
| 12 unrecognized character after (? or (?- |
| 13 POSIX named classes are supported only within a class |
| 14 missing ) |
| 15 reference to non-existent subpattern |
| 16 erroffset passed as NULL |
| 17 unknown option bit(s) set |
| 18 missing ) after comment |
| 19 [this code is not in use] |
| 20 regular expression is too large |
| 21 failed to get memory |
| 22 unmatched parentheses |
| 23 internal error: code overflow |
| 24 unrecognized character after (?< |
| 25 lookbehind assertion is not fixed length |
| 26 malformed number or name after (?( |
| 27 conditional group contains more than two branches |
| 28 assertion expected after (?( |
| 29 (?R or (?[+-]digits must be followed by ) |
| 30 unknown POSIX class name |
| 31 POSIX collating elements are not supported |
| 32 this version of PCRE is not compiled with PCRE_UTF8 support |
| 33 [this code is not in use] |
| 34 character value in \x{...} sequence is too large |
| 35 invalid condition (?(0) |
| 36 \C not allowed in lookbehind assertion |
| 37 PCRE does not support \L, \l, \N{name}, \U, or \u |
| 38 number after (?C is > 255 |
| 39 closing ) for (?C expected |
| 40 recursive call could loop indefinitely |
| 41 unrecognized character after (?P |
| 42 syntax error in subpattern name (missing terminator) |
| 43 two named subpatterns have the same name |
| 44 invalid UTF-8 string |
| 45 support for \P, \p, and \X has not been compiled |
| 46 malformed \P or \p sequence |
| 47 unknown property name after \P or \p |
| 48 subpattern name is too long (maximum 32 characters) |
| 49 too many named subpatterns (maximum 10000) |
| 50 [this code is not in use] |
| 51 octal value is greater than \377 (not in UTF-8 mode) |
| 52 internal error: overran compiling workspace |
| 53 internal error: previously-checked referenced subpattern |
| not found |
| 54 DEFINE group contains more than one branch |
| 55 repeating a DEFINE group is not allowed |
| 56 inconsistent NEWLINE options |
| 57 \g is not followed by a braced, angle-bracketed, or quoted |
| name/number or by a plain number |
| 58 a numbered reference must not be zero |
| 59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT) |
| 60 (*VERB) not recognized |
| 61 number is too big |
| 62 subpattern name expected |
| 63 digit expected after (?+ |
| 64 ] is an invalid data character in JavaScript compatibility mode |
| 65 different names for subpatterns of the same number are |
| not allowed |
| 66 (*MARK) must have an argument |
| 67 this version of PCRE is not compiled with PCRE_UCP support |
| 68 \c must be followed by an ASCII character |
| 69 \k is not followed by a braced, angle-bracketed, or quoted name |
| |
| The numbers 32 and 10000 in errors 48 and 49 are defaults; different |
| values may be used if the limits were changed when PCRE was built. |
| |
| |
| STUDYING A PATTERN |
| |
| pcre_extra *pcre_study(const pcre *code, int options |
| const char **errptr); |
| |
| If a compiled pattern is going to be used several times, it is worth |
| spending more time analyzing it in order to speed up the time taken for |
| matching. The function pcre_study() takes a pointer to a compiled pat- |
| tern as its first argument. If studying the pattern produces additional |
| information that will help speed up matching, pcre_study() returns a |
| pointer to a pcre_extra block, in which the study_data field points to |
| the results of the study. |
| |
| The returned value from pcre_study() can be passed directly to |
| pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con- |
| tains other fields that can be set by the caller before the block is |
| passed; these are described below in the section on matching a pattern. |
| |
| If studying the pattern does not produce any useful information, |
| pcre_study() returns NULL. In that circumstance, if the calling program |
| wants to pass any of the other fields to pcre_exec() or |
| pcre_dfa_exec(), it must set up its own pcre_extra block. |
| |
| The second argument of pcre_study() contains option bits. There is only |
| one option: PCRE_STUDY_JIT_COMPILE. If this is set, and the just-in- |
| time compiler is available, the pattern is further compiled into |
| machine code that executes much faster than the pcre_exec() matching |
| function. If the just-in-time compiler is not available, this option is |
| ignored. All other bits in the options argument must be zero. |
| |
| JIT compilation is a heavyweight optimization. It can take some time |
| for patterns to be analyzed, and for one-off matches and simple pat- |
| terns the benefit of faster execution might be offset by a much slower |
| study time. Not all patterns can be optimized by the JIT compiler. For |
| those that cannot be handled, matching automatically falls back to the |
| pcre_exec() interpreter. For more details, see the pcrejit documenta- |
| tion. |
| |
| The third argument for pcre_study() is a pointer for an error message. |
| If studying succeeds (even if no data is returned), the variable it |
| points to is set to NULL. Otherwise it is set to point to a textual |
| error message. This is a static string that is part of the library. You |
| must not try to free it. You should test the error pointer for NULL |
| after calling pcre_study(), to be sure that it has run successfully. |
| |
| When you are finished with a pattern, you can free the memory used for |
| the study data by calling pcre_free_study(). This function was added to |
| the API for release 8.20. For earlier versions, the memory could be |
| freed with pcre_free(), just like the pattern itself. This will still |
| work in cases where PCRE_STUDY_JIT_COMPILE is not used, but it is |
| advisable to change to the new function when convenient. |
| |
| This is a typical way in which pcre_study() is used (except that in a |
| real application there should be tests for errors): |
| |
| int rc; |
| pcre *re; |
| pcre_extra *sd; |
| re = pcre_compile("pattern", 0, &error, &erroroffset, NULL); |
| sd = pcre_study( |
| re, /* result of pcre_compile() */ |
| 0, /* no options */ |
| &error); /* set to NULL or points to a message */ |
| rc = pcre_exec( /* see below for details of pcre_exec() options */ |
| re, sd, "subject", 7, 0, 0, ovector, 30); |
| ... |
| pcre_free_study(sd); |
| pcre_free(re); |
| |
| Studying a pattern does two things: first, a lower bound for the length |
| of subject string that is needed to match the pattern is computed. This |
| does not mean that there are any strings of that length that match, but |
| it does guarantee that no shorter strings match. The value is used by |
| pcre_exec() and pcre_dfa_exec() to avoid wasting time by trying to |
| match strings that are shorter than the lower bound. You can find out |
| the value in a calling program via the pcre_fullinfo() function. |
| |
| Studying a pattern is also useful for non-anchored patterns that do not |
| have a single fixed starting character. A bitmap of possible starting |
| bytes is created. This speeds up finding a position in the subject at |
| which to start matching. |
| |
| These two optimizations apply to both pcre_exec() and pcre_dfa_exec(). |
| However, they are not used by pcre_exec() if pcre_study() is called |
| with the PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling is |
| successful. The optimizations can be disabled by setting the |
| PCRE_NO_START_OPTIMIZE option when calling pcre_exec() or |
| pcre_dfa_exec(). You might want to do this if your pattern contains |
| callouts or (*MARK) (which cannot be handled by the JIT compiler), and |
| you want to make use of these facilities in cases where matching fails. |
| See the discussion of PCRE_NO_START_OPTIMIZE below. |
| |
| |
| LOCALE SUPPORT |
| |
| PCRE handles caseless matching, and determines whether characters are |
| letters, digits, or whatever, by reference to a set of tables, indexed |
| by character value. When running in UTF-8 mode, this applies only to |
| characters with codes less than 128. By default, higher-valued codes |
| never match escapes such as \w or \d, but they can be tested with \p if |
| PCRE is built with Unicode character property support. Alternatively, |
| the PCRE_UCP option can be set at compile time; this causes \w and |
| friends to use Unicode property support instead of built-in tables. The |
| use of locales with Unicode is discouraged. If you are handling charac- |
| ters with codes greater than 128, you should either use UTF-8 and Uni- |
| code, or use locales, but not try to mix the two. |
| |
| PCRE contains an internal set of tables that are used when the final |
| argument of pcre_compile() is NULL. These are sufficient for many |
| applications. Normally, the internal tables recognize only ASCII char- |
| acters. However, when PCRE is built, it is possible to cause the inter- |
| nal tables to be rebuilt in the default "C" locale of the local system, |
| which may cause them to be different. |
| |
| The internal tables can always be overridden by tables supplied by the |
| application that calls PCRE. These may be created in a different locale |
| from the default. As more and more applications change to using Uni- |
| code, the need for this locale support is expected to die away. |
| |
| External tables are built by calling the pcre_maketables() function, |
| which has no arguments, in the relevant locale. The result can then be |
| passed to pcre_compile() or pcre_exec() as often as necessary. For |
| example, to build and use tables that are appropriate for the French |
| locale (where accented characters with values greater than 128 are |
| treated as letters), the following code could be used: |
| |
| setlocale(LC_CTYPE, "fr_FR"); |
| tables = pcre_maketables(); |
| re = pcre_compile(..., tables); |
| |
| The locale name "fr_FR" is used on Linux and other Unix-like systems; |
| if you are using Windows, the name for the French locale is "french". |
| |
| When pcre_maketables() runs, the tables are built in memory that is |
| obtained via pcre_malloc. It is the caller's responsibility to ensure |
| that the memory containing the tables remains available for as long as |
| it is needed. |
| |
| The pointer that is passed to pcre_compile() is saved with the compiled |
| pattern, and the same tables are used via this pointer by pcre_study() |
| and normally also by pcre_exec(). Thus, by default, for any single pat- |
| tern, compilation, studying and matching all happen in the same locale, |
| but different patterns can be compiled in different locales. |
| |
| It is possible to pass a table pointer or NULL (indicating the use of |
| the internal tables) to pcre_exec(). Although not intended for this |
| purpose, this facility could be used to match a pattern in a different |
| locale from the one in which it was compiled. Passing table pointers at |
| run time is discussed below in the section on matching a pattern. |
| |
| |
| INFORMATION ABOUT A PATTERN |
| |
| int pcre_fullinfo(const pcre *code, const pcre_extra *extra, |
| int what, void *where); |
| |
| The pcre_fullinfo() function returns information about a compiled pat- |
| tern. It replaces the obsolete pcre_info() function, which is neverthe- |
| less retained for backwards compability (and is documented below). |
| |
| The first argument for pcre_fullinfo() is a pointer to the compiled |
| pattern. The second argument is the result of pcre_study(), or NULL if |
| the pattern was not studied. The third argument specifies which piece |
| of information is required, and the fourth argument is a pointer to a |
| variable to receive the data. The yield of the function is zero for |
| success, or one of the following negative numbers: |
| |
| PCRE_ERROR_NULL the argument code was NULL |
| the argument where was NULL |
| PCRE_ERROR_BADMAGIC the "magic number" was not found |
| PCRE_ERROR_BADOPTION the value of what was invalid |
| |
| The "magic number" is placed at the start of each compiled pattern as |
| an simple check against passing an arbitrary memory pointer. Here is a |
| typical call of pcre_fullinfo(), to obtain the length of the compiled |
| pattern: |
| |
| int rc; |
| size_t length; |
| rc = pcre_fullinfo( |
| re, /* result of pcre_compile() */ |
| sd, /* result of pcre_study(), or NULL */ |
| PCRE_INFO_SIZE, /* what is required */ |
| &length); /* where to put the data */ |
| |
| The possible values for the third argument are defined in pcre.h, and |
| are as follows: |
| |
| PCRE_INFO_BACKREFMAX |
| |
| Return the number of the highest back reference in the pattern. The |
| fourth argument should point to an int variable. Zero is returned if |
| there are no back references. |
| |
| PCRE_INFO_CAPTURECOUNT |
| |
| Return the number of capturing subpatterns in the pattern. The fourth |
| argument should point to an int variable. |
| |
| PCRE_INFO_DEFAULT_TABLES |
| |
| Return a pointer to the internal default character tables within PCRE. |
| The fourth argument should point to an unsigned char * variable. This |
| information call is provided for internal use by the pcre_study() func- |
| tion. External callers can cause PCRE to use its internal tables by |
| passing a NULL table pointer. |
| |
| PCRE_INFO_FIRSTBYTE |
| |
| Return information about the first byte of any matched string, for a |
| non-anchored pattern. The fourth argument should point to an int vari- |
| able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name |
| is still recognized for backwards compatibility.) |
| |
| If there is a fixed first byte, for example, from a pattern such as |
| (cat|cow|coyote), its value is returned. Otherwise, if either |
| |
| (a) the pattern was compiled with the PCRE_MULTILINE option, and every |
| branch starts with "^", or |
| |
| (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not |
| set (if it were set, the pattern would be anchored), |
| |
| -1 is returned, indicating that the pattern matches only at the start |
| of a subject string or after any newline within the string. Otherwise |
| -2 is returned. For anchored patterns, -2 is returned. |
| |
| PCRE_INFO_FIRSTTABLE |
| |
| If the pattern was studied, and this resulted in the construction of a |
| 256-bit table indicating a fixed set of bytes for the first byte in any |
| matching string, a pointer to the table is returned. Otherwise NULL is |
| returned. The fourth argument should point to an unsigned char * vari- |
| able. |
| |
| PCRE_INFO_HASCRORLF |
| |
| Return 1 if the pattern contains any explicit matches for CR or LF |
| characters, otherwise 0. The fourth argument should point to an int |
| variable. An explicit match is either a literal CR or LF character, or |
| \r or \n. |
| |
| PCRE_INFO_JCHANGED |
| |
| Return 1 if the (?J) or (?-J) option setting is used in the pattern, |
| otherwise 0. The fourth argument should point to an int variable. (?J) |
| and (?-J) set and unset the local PCRE_DUPNAMES option, respectively. |
| |
| PCRE_INFO_JIT |
| |
| Return 1 if the pattern was studied with the PCRE_STUDY_JIT_COMPILE |
| option, and just-in-time compiling was successful. The fourth argument |
| should point to an int variable. A return value of 0 means that JIT |
| support is not available in this version of PCRE, or that the pattern |
| was not studied with the PCRE_STUDY_JIT_COMPILE option, or that the JIT |
| compiler could not handle this particular pattern. See the pcrejit doc- |
| umentation for details of what can and cannot be handled. |
| |
| PCRE_INFO_JITSIZE |
| |
| If the pattern was successfully studied with the PCRE_STUDY_JIT_COMPILE |
| option, return the size of the JIT compiled code, otherwise return |
| zero. The fourth argument should point to a size_t variable. |
| |
| PCRE_INFO_LASTLITERAL |
| |
| Return the value of the rightmost literal byte that must exist in any |
| matched string, other than at its start, if such a byte has been |
| recorded. The fourth argument should point to an int variable. If there |
| is no such byte, -1 is returned. For anchored patterns, a last literal |
| byte is recorded only if it follows something of variable length. For |
| example, for the pattern /^a\d+z\d+/ the returned value is "z", but for |
| /^a\dz\d/ the returned value is -1. |
| |
| PCRE_INFO_MINLENGTH |
| |
| If the pattern was studied and a minimum length for matching subject |
| strings was computed, its value is returned. Otherwise the returned |
| value is -1. The value is a number of characters, not bytes (this may |
| be relevant in UTF-8 mode). The fourth argument should point to an int |
| variable. A non-negative value is a lower bound to the length of any |
| matching string. There may not be any strings of that length that do |
| actually match, but every string that does match is at least that long. |
| |
| PCRE_INFO_NAMECOUNT |
| PCRE_INFO_NAMEENTRYSIZE |
| PCRE_INFO_NAMETABLE |
| |
| PCRE supports the use of named as well as numbered capturing parenthe- |
| ses. The names are just an additional way of identifying the parenthe- |
| ses, which still acquire numbers. Several convenience functions such as |
| pcre_get_named_substring() are provided for extracting captured sub- |
| strings by name. It is also possible to extract the data directly, by |
| first converting the name to a number in order to access the correct |
| pointers in the output vector (described with pcre_exec() below). To do |
| the conversion, you need to use the name-to-number map, which is |
| described by these three values. |
| |
| The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT |
| gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size |
| of each entry; both of these return an int value. The entry size |
| depends on the length of the longest name. PCRE_INFO_NAMETABLE returns |
| a pointer to the first entry of the table (a pointer to char). The |
| first two bytes of each entry are the number of the capturing parenthe- |
| sis, most significant byte first. The rest of the entry is the corre- |
| sponding name, zero terminated. |
| |
| The names are in alphabetical order. Duplicate names may appear if (?| |
| is used to create multiple groups with the same number, as described in |
| the section on duplicate subpattern numbers in the pcrepattern page. |
| Duplicate names for subpatterns with different numbers are permitted |
| only if PCRE_DUPNAMES is set. In all cases of duplicate names, they |
| appear in the table in the order in which they were found in the pat- |
| tern. In the absence of (?| this is the order of increasing number; |
| when (?| is used this is not necessarily the case because later subpat- |
| terns may have lower numbers. |
| |
| As a simple example of the name/number table, consider the following |
| pattern (assume PCRE_EXTENDED is set, so white space - including new- |
| lines - is ignored): |
| |
| (?<date> (?<year>(\d\d)?\d\d) - |
| (?<month>\d\d) - (?<day>\d\d) ) |
| |
| There are four named subpatterns, so the table has four entries, and |
| each entry in the table is eight bytes long. The table is as follows, |
| with non-printing bytes shows in hexadecimal, and undefined bytes shown |
| as ??: |
| |
| 00 01 d a t e 00 ?? |
| 00 05 d a y 00 ?? ?? |
| 00 04 m o n t h 00 |
| 00 02 y e a r 00 ?? |
| |
| When writing code to extract data from named subpatterns using the |
| name-to-number map, remember that the length of the entries is likely |
| to be different for each compiled pattern. |
| |
| PCRE_INFO_OKPARTIAL |
| |
| Return 1 if the pattern can be used for partial matching with |
| pcre_exec(), otherwise 0. The fourth argument should point to an int |
| variable. From release 8.00, this always returns 1, because the |
| restrictions that previously applied to partial matching have been |
| lifted. The pcrepartial documentation gives details of partial match- |
| ing. |
| |
| PCRE_INFO_OPTIONS |
| |
| Return a copy of the options with which the pattern was compiled. The |
| fourth argument should point to an unsigned long int variable. These |
| option bits are those specified in the call to pcre_compile(), modified |
| by any top-level option settings at the start of the pattern itself. In |
| other words, they are the options that will be in force when matching |
| starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with |
| the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE, |
| and PCRE_EXTENDED. |
| |
| A pattern is automatically anchored by PCRE if all of its top-level |
| alternatives begin with one of the following: |
| |
| ^ unless PCRE_MULTILINE is set |
| \A always |
| \G always |
| .* if PCRE_DOTALL is set and there are no back |
| references to the subpattern in which .* appears |
| |
| For such patterns, the PCRE_ANCHORED bit is set in the options returned |
| by pcre_fullinfo(). |
| |
| PCRE_INFO_SIZE |
| |
| Return the size of the compiled pattern. The fourth argument should |
| point to a size_t variable. This value does not include the size of the |
| pcre structure that is returned by pcre_compile(). The value that is |
| passed as the argument to pcre_malloc() when pcre_compile() is getting |
| memory in which to place the compiled data is the value returned by |
| this option plus the size of the pcre structure. Studying a compiled |
| pattern, with or without JIT, does not alter the value returned by this |
| option. |
| |
| PCRE_INFO_STUDYSIZE |
| |
| Return the size of the data block pointed to by the study_data field in |
| a pcre_extra block. If pcre_extra is NULL, or there is no study data, |
| zero is returned. The fourth argument should point to a size_t vari- |
| able. The study_data field is set by pcre_study() to record informa- |
| tion that will speed up matching (see the section entitled "Studying a |
| pattern" above). The format of the study_data block is private, but its |
| length is made available via this option so that it can be saved and |
| restored (see the pcreprecompile documentation for details). |
| |
| |
| OBSOLETE INFO FUNCTION |
| |
| int pcre_info(const pcre *code, int *optptr, int *firstcharptr); |
| |
| The pcre_info() function is now obsolete because its interface is too |
| restrictive to return all the available data about a compiled pattern. |
| New programs should use pcre_fullinfo() instead. The yield of |
| pcre_info() is the number of capturing subpatterns, or one of the fol- |
| lowing negative numbers: |
| |
| PCRE_ERROR_NULL the argument code was NULL |
| PCRE_ERROR_BADMAGIC the "magic number" was not found |
| |
| If the optptr argument is not NULL, a copy of the options with which |
| the pattern was compiled is placed in the integer it points to (see |
| PCRE_INFO_OPTIONS above). |
| |
| If the pattern is not anchored and the firstcharptr argument is not |
| NULL, it is used to pass back information about the first character of |
| any matched string (see PCRE_INFO_FIRSTBYTE above). |
| |
| |
| REFERENCE COUNTS |
| |
| int pcre_refcount(pcre *code, int adjust); |
| |
| The pcre_refcount() function is used to maintain a reference count in |
| the data block that contains a compiled pattern. It is provided for the |
| benefit of applications that operate in an object-oriented manner, |
| where different parts of the application may be using the same compiled |
| pattern, but you want to free the block when they are all done. |
| |
| When a pattern is compiled, the reference count field is initialized to |
| zero. It is changed only by calling this function, whose action is to |
| add the adjust value (which may be positive or negative) to it. The |
| yield of the function is the new value. However, the value of the count |
| is constrained to lie between 0 and 65535, inclusive. If the new value |
| is outside these limits, it is forced to the appropriate limit value. |
| |
| Except when it is zero, the reference count is not correctly preserved |
| if a pattern is compiled on one host and then transferred to a host |
| whose byte-order is different. (This seems a highly unlikely scenario.) |
| |
| |
| MATCHING A PATTERN: THE TRADITIONAL FUNCTION |
| |
| int pcre_exec(const pcre *code, const pcre_extra *extra, |
| const char *subject, int length, int startoffset, |
| int options, int *ovector, int ovecsize); |
| |
| The function pcre_exec() is called to match a subject string against a |
| compiled pattern, which is passed in the code argument. If the pattern |
| was studied, the result of the study should be passed in the extra |
| argument. You can call pcre_exec() with the same code and extra argu- |
| ments as many times as you like, in order to match different subject |
| strings with the same pattern. |
| |
| This function is the main matching facility of the library, and it |
| operates in a Perl-like manner. For specialist use there is also an |
| alternative matching function, which is described below in the section |
| about the pcre_dfa_exec() function. |
| |
| In most applications, the pattern will have been compiled (and option- |
| ally studied) in the same process that calls pcre_exec(). However, it |
| is possible to save compiled patterns and study data, and then use them |
| later in different processes, possibly even on different hosts. For a |
| discussion about this, see the pcreprecompile documentation. |
| |
| Here is an example of a simple call to pcre_exec(): |
| |
| int rc; |
| int ovector[30]; |
| rc = pcre_exec( |
| re, /* result of pcre_compile() */ |
| NULL, /* we didn't study the pattern */ |
| "some string", /* the subject string */ |
| 11, /* the length of the subject string */ |
| 0, /* start at offset 0 in the subject */ |
| 0, /* default options */ |
| ovector, /* vector of integers for substring information */ |
| 30); /* number of elements (NOT size in bytes) */ |
| |
| Extra data for pcre_exec() |
| |
| If the extra argument is not NULL, it must point to a pcre_extra data |
| block. The pcre_study() function returns such a block (when it doesn't |
| return NULL), but you can also create one for yourself, and pass addi- |
| tional information in it. The pcre_extra block contains the following |
| fields (not necessarily in this order): |
| |
| unsigned long int flags; |
| void *study_data; |
| void *executable_jit; |
| unsigned long int match_limit; |
| unsigned long int match_limit_recursion; |
| void *callout_data; |
| const unsigned char *tables; |
| unsigned char **mark; |
| |
| The flags field is a bitmap that specifies which of the other fields |
| are set. The flag bits are: |
| |
| PCRE_EXTRA_STUDY_DATA |
| PCRE_EXTRA_EXECUTABLE_JIT |
| PCRE_EXTRA_MATCH_LIMIT |
| PCRE_EXTRA_MATCH_LIMIT_RECURSION |
| PCRE_EXTRA_CALLOUT_DATA |
| PCRE_EXTRA_TABLES |
| PCRE_EXTRA_MARK |
| |
| Other flag bits should be set to zero. The study_data field and some- |
| times the executable_jit field are set in the pcre_extra block that is |
| returned by pcre_study(), together with the appropriate flag bits. You |
| should not set these yourself, but you may add to the block by setting |
| the other fields and their corresponding flag bits. |
| |
| The match_limit field provides a means of preventing PCRE from using up |
| a vast amount of resources when running patterns that are not going to |
| match, but which have a very large number of possibilities in their |
| search trees. The classic example is a pattern that uses nested unlim- |
| ited repeats. |
| |
| Internally, pcre_exec() uses a function called match(), which it calls |
| repeatedly (sometimes recursively). The limit set by match_limit is |
| imposed on the number of times this function is called during a match, |
| which has the effect of limiting the amount of backtracking that can |
| take place. For patterns that are not anchored, the count restarts from |
| zero for each position in the subject string. |
| |
| When pcre_exec() is called with a pattern that was successfully studied |
| with the PCRE_STUDY_JIT_COMPILE option, the way that the matching is |
| executed is entirely different. However, there is still the possibility |
| of runaway matching that goes on for a very long time, and so the |
| match_limit value is also used in this case (but in a different way) to |
| limit how long the matching can continue. |
| |
| The default value for the limit can be set when PCRE is built; the |
| default default is 10 million, which handles all but the most extreme |
| cases. You can override the default by suppling pcre_exec() with a |
| pcre_extra block in which match_limit is set, and |
| PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is |
| exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT. |
| |
| The match_limit_recursion field is similar to match_limit, but instead |
| of limiting the total number of times that match() is called, it limits |
| the depth of recursion. The recursion depth is a smaller number than |
| the total number of calls, because not all calls to match() are recur- |
| sive. This limit is of use only if it is set smaller than match_limit. |
| |
| Limiting the recursion depth limits the amount of machine stack that |
| can be used, or, when PCRE has been compiled to use memory on the heap |
| instead of the stack, the amount of heap memory that can be used. This |
| limit is not relevant, and is ignored, if the pattern was successfully |
| studied with PCRE_STUDY_JIT_COMPILE. |
| |
| The default value for match_limit_recursion can be set when PCRE is |
| built; the default default is the same value as the default for |
| match_limit. You can override the default by suppling pcre_exec() with |
| a pcre_extra block in which match_limit_recursion is set, and |
| PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the |
| limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT. |
| |
| The callout_data field is used in conjunction with the "callout" fea- |
| ture, and is described in the pcrecallout documentation. |
| |
| The tables field is used to pass a character tables pointer to |
| pcre_exec(); this overrides the value that is stored with the compiled |
| pattern. A non-NULL value is stored with the compiled pattern only if |
| custom tables were supplied to pcre_compile() via its tableptr argu- |
| ment. If NULL is passed to pcre_exec() using this mechanism, it forces |
| PCRE's internal tables to be used. This facility is helpful when re- |
| using patterns that have been saved after compiling with an external |
| set of tables, because the external tables might be at a different |
| address when pcre_exec() is called. See the pcreprecompile documenta- |
| tion for a discussion of saving compiled patterns for later use. |
| |
| If PCRE_EXTRA_MARK is set in the flags field, the mark field must be |
| set to point to a char * variable. If the pattern contains any back- |
| tracking control verbs such as (*MARK:NAME), and the execution ends up |
| with a name to pass back, a pointer to the name string (zero termi- |
| nated) is placed in the variable pointed to by the mark field. The |
| names are within the compiled pattern; if you wish to retain such a |
| name you must copy it before freeing the memory of a compiled pattern. |
| If there is no name to pass back, the variable pointed to by the mark |
| field set to NULL. For details of the backtracking control verbs, see |
| the section entitled "Backtracking control" in the pcrepattern documen- |
| tation. |
| |
| Option bits for pcre_exec() |
| |
| The unused bits of the options argument for pcre_exec() must be zero. |
| The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx, |
| PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, |
| PCRE_NO_START_OPTIMIZE, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_SOFT, and |
| PCRE_PARTIAL_HARD. |
| |
| If the pattern was successfully studied with the PCRE_STUDY_JIT_COMPILE |
| option, the only supported options for JIT execution are |
| PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and |
| PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not |
| supported. If an unsupported option is used, JIT execution is disabled |
| and the normal interpretive code in pcre_exec() is run. |
| |
| PCRE_ANCHORED |
| |
| The PCRE_ANCHORED option limits pcre_exec() to matching at the first |
| matching position. If a pattern was compiled with PCRE_ANCHORED, or |
| turned out to be anchored by virtue of its contents, it cannot be made |
| unachored at matching time. |
| |
| PCRE_BSR_ANYCRLF |
| PCRE_BSR_UNICODE |
| |
| These options (which are mutually exclusive) control what the \R escape |
| sequence matches. The choice is either to match only CR, LF, or CRLF, |
| or to match any Unicode newline sequence. These options override the |
| choice that was made or defaulted when the pattern was compiled. |
| |
| PCRE_NEWLINE_CR |
| PCRE_NEWLINE_LF |
| PCRE_NEWLINE_CRLF |
| PCRE_NEWLINE_ANYCRLF |
| PCRE_NEWLINE_ANY |
| |
| These options override the newline definition that was chosen or |
| defaulted when the pattern was compiled. For details, see the descrip- |
| tion of pcre_compile() above. During matching, the newline choice |
| affects the behaviour of the dot, circumflex, and dollar metacharac- |
| ters. It may also alter the way the match position is advanced after a |
| match failure for an unanchored pattern. |
| |
| When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is |
| set, and a match attempt for an unanchored pattern fails when the cur- |
| rent position is at a CRLF sequence, and the pattern contains no |
| explicit matches for CR or LF characters, the match position is |
| advanced by two characters instead of one, in other words, to after the |
| CRLF. |
| |
| The above rule is a compromise that makes the most common cases work as |
| expected. For example, if the pattern is .+A (and the PCRE_DOTALL |
| option is not set), it does not match the string "\r\nA" because, after |
| failing at the start, it skips both the CR and the LF before retrying. |
| However, the pattern [\r\n]A does match that string, because it con- |
| tains an explicit CR or LF reference, and so advances only by one char- |
| acter after the first failure. |
| |
| An explicit match for CR of LF is either a literal appearance of one of |
| those characters, or one of the \r or \n escape sequences. Implicit |
| matches such as [^X] do not count, nor does \s (which includes CR and |
| LF in the characters that it matches). |
| |
| Notwithstanding the above, anomalous effects may still occur when CRLF |
| is a valid newline sequence and explicit \r or \n escapes appear in the |
| pattern. |
| |
| PCRE_NOTBOL |
| |
| This option specifies that first character of the subject string is not |
| the beginning of a line, so the circumflex metacharacter should not |
| match before it. Setting this without PCRE_MULTILINE (at compile time) |
| causes circumflex never to match. This option affects only the behav- |
| iour of the circumflex metacharacter. It does not affect \A. |
| |
| PCRE_NOTEOL |
| |
| This option specifies that the end of the subject string is not the end |
| of a line, so the dollar metacharacter should not match it nor (except |
| in multiline mode) a newline immediately before it. Setting this with- |
| out PCRE_MULTILINE (at compile time) causes dollar never to match. This |
| option affects only the behaviour of the dollar metacharacter. It does |
| not affect \Z or \z. |
| |
| PCRE_NOTEMPTY |
| |
| An empty string is not considered to be a valid match if this option is |
| set. If there are alternatives in the pattern, they are tried. If all |
| the alternatives match the empty string, the entire match fails. For |
| example, if the pattern |
| |
| a?b? |
| |
| is applied to a string not beginning with "a" or "b", it matches an |
| empty string at the start of the subject. With PCRE_NOTEMPTY set, this |
| match is not valid, so PCRE searches further into the string for occur- |
| rences of "a" or "b". |
| |
| PCRE_NOTEMPTY_ATSTART |
| |
| This is like PCRE_NOTEMPTY, except that an empty string match that is |
| not at the start of the subject is permitted. If the pattern is |
| anchored, such a match can occur only if the pattern contains \K. |
| |
| Perl has no direct equivalent of PCRE_NOTEMPTY or |
| PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern |
| match of the empty string within its split() function, and when using |
| the /g modifier. It is possible to emulate Perl's behaviour after |
| matching a null string by first trying the match again at the same off- |
| set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that |
| fails, by advancing the starting offset (see below) and trying an ordi- |
| nary match again. There is some code that demonstrates how to do this |
| in the pcredemo sample program. In the most general case, you have to |
| check to see if the newline convention recognizes CRLF as a newline, |
| and if so, and the current character is CR followed by LF, advance the |
| starting offset by two characters instead of one. |
| |
| PCRE_NO_START_OPTIMIZE |
| |
| There are a number of optimizations that pcre_exec() uses at the start |
| of a match, in order to speed up the process. For example, if it is |
| known that an unanchored match must start with a specific character, it |
| searches the subject for that character, and fails immediately if it |
| cannot find it, without actually running the main matching function. |
| This means that a special item such as (*COMMIT) at the start of a pat- |
| tern is not considered until after a suitable starting point for the |
| match has been found. When callouts or (*MARK) items are in use, these |
| "start-up" optimizations can cause them to be skipped if the pattern is |
| never actually used. The start-up optimizations are in effect a pre- |
| scan of the subject that takes place before the pattern is run. |
| |
| The PCRE_NO_START_OPTIMIZE option disables the start-up optimizations, |
| possibly causing performance to suffer, but ensuring that in cases |
| where the result is "no match", the callouts do occur, and that items |
| such as (*COMMIT) and (*MARK) are considered at every possible starting |
| position in the subject string. If PCRE_NO_START_OPTIMIZE is set at |
| compile time, it cannot be unset at matching time. |
| |
| Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching |
| operation. Consider the pattern |
| |
| (*COMMIT)ABC |
| |
| When this is compiled, PCRE records the fact that a match must start |
| with the character "A". Suppose the subject string is "DEFABC". The |
| start-up optimization scans along the subject, finds "A" and runs the |
| first match attempt from there. The (*COMMIT) item means that the pat- |
| tern must match the current starting position, which in this case, it |
| does. However, if the same match is run with PCRE_NO_START_OPTIMIZE |
| set, the initial scan along the subject string does not happen. The |
| first match attempt is run starting from "D" and when this fails, |
| (*COMMIT) prevents any further matches being tried, so the overall |
| result is "no match". If the pattern is studied, more start-up opti- |
| mizations may be used. For example, a minimum length for the subject |
| may be recorded. Consider the pattern |
| |
| (*MARK:A)(X|Y) |
| |
| The minimum length for a match is one character. If the subject is |
| "ABC", there will be attempts to match "ABC", "BC", "C", and then |
| finally an empty string. If the pattern is studied, the final attempt |
| does not take place, because PCRE knows that the subject is too short, |
| and so the (*MARK) is never encountered. In this case, studying the |
| pattern does not affect the overall match result, which is still "no |
| match", but it does affect the auxiliary information that is returned. |
| |
| PCRE_NO_UTF8_CHECK |
| |
| When PCRE_UTF8 is set at compile time, the validity of the subject as a |
| UTF-8 string is automatically checked when pcre_exec() is subsequently |
| called. The value of startoffset is also checked to ensure that it |
| points to the start of a UTF-8 character. There is a discussion about |
| the validity of UTF-8 strings in the section on UTF-8 support in the |
| main pcre page. If an invalid UTF-8 sequence of bytes is found, |
| pcre_exec() returns the error PCRE_ERROR_BADUTF8 or, if PCRE_PAR- |
| TIAL_HARD is set and the problem is a truncated UTF-8 character at the |
| end of the subject, PCRE_ERROR_SHORTUTF8. In both cases, information |
| about the precise nature of the error may also be returned (see the |
| descriptions of these errors in the section entitled Error return val- |
| ues from pcre_exec() below). If startoffset contains a value that does |
| not point to the start of a UTF-8 character (or to the end of the sub- |
| ject), PCRE_ERROR_BADUTF8_OFFSET is returned. |
| |
| If you already know that your subject is valid, and you want to skip |
| these checks for performance reasons, you can set the |
| PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to |
| do this for the second and subsequent calls to pcre_exec() if you are |
| making repeated calls to find all the matches in a single subject |
| string. However, you should be sure that the value of startoffset |
| points to the start of a UTF-8 character (or the end of the subject). |
| When PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid UTF-8 |
| string as a subject or an invalid value of startoffset is undefined. |
| Your program may crash. |
| |
| PCRE_PARTIAL_HARD |
| PCRE_PARTIAL_SOFT |
| |
| These options turn on the partial matching feature. For backwards com- |
| patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial |
| match occurs if the end of the subject string is reached successfully, |
| but there are not enough subject characters to complete the match. If |
| this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set, |
| matching continues by testing any remaining alternatives. Only if no |
| complete match can be found is PCRE_ERROR_PARTIAL returned instead of |
| PCRE_ERROR_NOMATCH. In other words, PCRE_PARTIAL_SOFT says that the |
| caller is prepared to handle a partial match, but only if no complete |
| match can be found. |
| |
| If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this |
| case, if a partial match is found, pcre_exec() immediately returns |
| PCRE_ERROR_PARTIAL, without considering any other alternatives. In |
| other words, when PCRE_PARTIAL_HARD is set, a partial match is consid- |
| ered to be more important that an alternative complete match. |
| |
| In both cases, the portion of the string that was inspected when the |
| partial match was found is set as the first matching string. There is a |
| more detailed discussion of partial and multi-segment matching, with |
| examples, in the pcrepartial documentation. |
| |
| The string to be matched by pcre_exec() |
| |
| The subject string is passed to pcre_exec() as a pointer in subject, a |
| length (in bytes) in length, and a starting byte offset in startoffset. |
| If this is negative or greater than the length of the subject, |
| pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is |
| zero, the search for a match starts at the beginning of the subject, |
| and this is by far the most common case. In UTF-8 mode, the byte offset |
| must point to the start of a UTF-8 character (or the end of the sub- |
| ject). Unlike the pattern string, the subject may contain binary zero |
| bytes. |
| |
| A non-zero starting offset is useful when searching for another match |
| in the same subject by calling pcre_exec() again after a previous suc- |
| cess. Setting startoffset differs from just passing over a shortened |
| string and setting PCRE_NOTBOL in the case of a pattern that begins |
| with any kind of lookbehind. For example, consider the pattern |
| |
| \Biss\B |
| |
| which finds occurrences of "iss" in the middle of words. (\B matches |
| only if the current position in the subject is not a word boundary.) |
| When applied to the string "Mississipi" the first call to pcre_exec() |
| finds the first occurrence. If pcre_exec() is called again with just |
| the remainder of the subject, namely "issipi", it does not match, |
| because \B is always false at the start of the subject, which is deemed |
| to be a word boundary. However, if pcre_exec() is passed the entire |
| string again, but with startoffset set to 4, it finds the second occur- |
| rence of "iss" because it is able to look behind the starting point to |
| discover that it is preceded by a letter. |
| |
| Finding all the matches in a subject is tricky when the pattern can |
| match an empty string. It is possible to emulate Perl's /g behaviour by |
| first trying the match again at the same offset, with the |
| PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that |
| fails, advancing the starting offset and trying an ordinary match |
| again. There is some code that demonstrates how to do this in the pcre- |
| demo sample program. In the most general case, you have to check to see |
| if the newline convention recognizes CRLF as a newline, and if so, and |
| the current character is CR followed by LF, advance the starting offset |
| by two characters instead of one. |
| |
| If a non-zero starting offset is passed when the pattern is anchored, |
| one attempt to match at the given offset is made. This can only succeed |
| if the pattern does not require the match to be at the start of the |
| subject. |
| |
| How pcre_exec() returns captured substrings |
| |
| In general, a pattern matches a certain portion of the subject, and in |
| addition, further substrings from the subject may be picked out by |
| parts of the pattern. Following the usage in Jeffrey Friedl's book, |
| this is called "capturing" in what follows, and the phrase "capturing |
| subpattern" is used for a fragment of a pattern that picks out a sub- |
| string. PCRE supports several other kinds of parenthesized subpattern |
| that do not cause substrings to be captured. |
| |
| Captured substrings are returned to the caller via a vector of integers |
| whose address is passed in ovector. The number of elements in the vec- |
| tor is passed in ovecsize, which must be a non-negative number. Note: |
| this argument is NOT the size of ovector in bytes. |
| |
| The first two-thirds of the vector is used to pass back captured sub- |
| strings, each substring using a pair of integers. The remaining third |
| of the vector is used as workspace by pcre_exec() while matching cap- |
| turing subpatterns, and is not available for passing back information. |
| The number passed in ovecsize should always be a multiple of three. If |
| it is not, it is rounded down. |
| |
| When a match is successful, information about captured substrings is |
| returned in pairs of integers, starting at the beginning of ovector, |
| and continuing up to two-thirds of its length at the most. The first |
| element of each pair is set to the byte offset of the first character |
| in a substring, and the second is set to the byte offset of the first |
| character after the end of a substring. Note: these values are always |
| byte offsets, even in UTF-8 mode. They are not character counts. |
| |
| The first pair of integers, ovector[0] and ovector[1], identify the |
| portion of the subject string matched by the entire pattern. The next |
| pair is used for the first capturing subpattern, and so on. The value |
| returned by pcre_exec() is one more than the highest numbered pair that |
| has been set. For example, if two substrings have been captured, the |
| returned value is 3. If there are no capturing subpatterns, the return |
| value from a successful match is 1, indicating that just the first pair |
| of offsets has been set. |
| |
| If a capturing subpattern is matched repeatedly, it is the last portion |
| of the string that it matched that is returned. |
| |
| If the vector is too small to hold all the captured substring offsets, |
| it is used as far as possible (up to two-thirds of its length), and the |
| function returns a value of zero. If neither the actual string matched |
| not any captured substrings are of interest, pcre_exec() may be called |
| with ovector passed as NULL and ovecsize as zero. However, if the pat- |
| tern contains back references and the ovector is not big enough to |
| remember the related substrings, PCRE has to get additional memory for |
| use during matching. Thus it is usually advisable to supply an ovector |
| of reasonable size. |
| |
| There are some cases where zero is returned (indicating vector over- |
| flow) when in fact the vector is exactly the right size for the final |
| match. For example, consider the pattern |
| |
| (a)(?:(b)c|bd) |
| |
| If a vector of 6 elements (allowing for only 1 captured substring) is |
| given with subject string "abd", pcre_exec() will try to set the second |
| captured string, thereby recording a vector overflow, before failing to |
| match "c" and backing up to try the second alternative. The zero |
| return, however, does correctly indicate that the maximum number of |
| slots (namely 2) have been filled. In similar cases where there is tem- |
| porary overflow, but the final number of used slots is actually less |
| than the maximum, a non-zero value is returned. |
| |
| The pcre_fullinfo() function can be used to find out how many capturing |
| subpatterns there are in a compiled pattern. The smallest size for |
| ovector that will allow for n captured substrings, in addition to the |
| offsets of the substring matched by the whole pattern, is (n+1)*3. |
| |
| It is possible for capturing subpattern number n+1 to match some part |
| of the subject when subpattern n has not been used at all. For example, |
| if the string "abc" is matched against the pattern (a|(z))(bc) the |
| return from the function is 4, and subpatterns 1 and 3 are matched, but |
| 2 is not. When this happens, both values in the offset pairs corre- |
| sponding to unused subpatterns are set to -1. |
| |
| Offset values that correspond to unused subpatterns at the end of the |
| expression are also set to -1. For example, if the string "abc" is |
| matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not |
| matched. The return from the function is 2, because the highest used |
| capturing subpattern number is 1, and the offsets for for the second |
| and third capturing subpatterns (assuming the vector is large enough, |
| of course) are set to -1. |
| |
| Note: Elements in the first two-thirds of ovector that do not corre- |
| spond to capturing parentheses in the pattern are never changed. That |
| is, if a pattern contains n capturing parentheses, no more than ovec- |
| tor[0] to ovector[2n+1] are set by pcre_exec(). The other elements (in |
| the first two-thirds) retain whatever values they previously had. |
| |
| Some convenience functions are provided for extracting the captured |
| substrings as separate strings. These are described below. |
| |
| Error return values from pcre_exec() |
| |
| If pcre_exec() fails, it returns a negative number. The following are |
| defined in the header file: |
| |
| PCRE_ERROR_NOMATCH (-1) |
| |
| The subject string did not match the pattern. |
| |
| PCRE_ERROR_NULL (-2) |
| |
| Either code or subject was passed as NULL, or ovector was NULL and |
| ovecsize was not zero. |
| |
| PCRE_ERROR_BADOPTION (-3) |
| |
| An unrecognized bit was set in the options argument. |
| |
| PCRE_ERROR_BADMAGIC (-4) |
| |
| PCRE stores a 4-byte "magic number" at the start of the compiled code, |
| to catch the case when it is passed a junk pointer and to detect when a |
| pattern that was compiled in an environment of one endianness is run in |
| an environment with the other endianness. This is the error that PCRE |
| gives when the magic number is not present. |
| |
| PCRE_ERROR_UNKNOWN_OPCODE (-5) |
| |
| While running the pattern match, an unknown item was encountered in the |
| compiled pattern. This error could be caused by a bug in PCRE or by |
| overwriting of the compiled pattern. |
| |
| PCRE_ERROR_NOMEMORY (-6) |
| |
| If a pattern contains back references, but the ovector that is passed |
| to pcre_exec() is not big enough to remember the referenced substrings, |
| PCRE gets a block of memory at the start of matching to use for this |
| purpose. If the call via pcre_malloc() fails, this error is given. The |
| memory is automatically freed at the end of matching. |
| |
| This error is also given if pcre_stack_malloc() fails in pcre_exec(). |
| This can happen only when PCRE has been compiled with --disable-stack- |
| for-recursion. |
| |
| PCRE_ERROR_NOSUBSTRING (-7) |
| |
| This error is used by the pcre_copy_substring(), pcre_get_substring(), |
| and pcre_get_substring_list() functions (see below). It is never |
| returned by pcre_exec(). |
| |
| PCRE_ERROR_MATCHLIMIT (-8) |
| |
| The backtracking limit, as specified by the match_limit field in a |
| pcre_extra structure (or defaulted) was reached. See the description |
| above. |
| |
| PCRE_ERROR_CALLOUT (-9) |
| |
| This error is never generated by pcre_exec() itself. It is provided for |
| use by callout functions that want to yield a distinctive error code. |
| See the pcrecallout documentation for details. |
| |
| PCRE_ERROR_BADUTF8 (-10) |
| |
| A string that contains an invalid UTF-8 byte sequence was passed as a |
| subject, and the PCRE_NO_UTF8_CHECK option was not set. If the size of |
| the output vector (ovecsize) is at least 2, the byte offset to the |
| start of the the invalid UTF-8 character is placed in the first ele- |
| ment, and a reason code is placed in the second element. The reason |
| codes are listed in the following section. For backward compatibility, |
| if PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 char- |
| acter at the end of the subject (reason codes 1 to 5), |
| PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8. |
| |
| PCRE_ERROR_BADUTF8_OFFSET (-11) |
| |
| The UTF-8 byte sequence that was passed as a subject was checked and |
| found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but the |
| value of startoffset did not point to the beginning of a UTF-8 charac- |
| ter or the end of the subject. |
| |
| PCRE_ERROR_PARTIAL (-12) |
| |
| The subject string did not match, but it did match partially. See the |
| pcrepartial documentation for details of partial matching. |
| |
| PCRE_ERROR_BADPARTIAL (-13) |
| |
| This code is no longer in use. It was formerly returned when the |
| PCRE_PARTIAL option was used with a compiled pattern containing items |
| that were not supported for partial matching. From release 8.00 |
| onwards, there are no restrictions on partial matching. |
| |
| PCRE_ERROR_INTERNAL (-14) |
| |
| An unexpected internal error has occurred. This error could be caused |
| by a bug in PCRE or by overwriting of the compiled pattern. |
| |
| PCRE_ERROR_BADCOUNT (-15) |
| |
| This error is given if the value of the ovecsize argument is negative. |
| |
| PCRE_ERROR_RECURSIONLIMIT (-21) |
| |
| The internal recursion limit, as specified by the match_limit_recursion |
| field in a pcre_extra structure (or defaulted) was reached. See the |
| description above. |
| |
| PCRE_ERROR_BADNEWLINE (-23) |
| |
| An invalid combination of PCRE_NEWLINE_xxx options was given. |
| |
| PCRE_ERROR_BADOFFSET (-24) |
| |
| The value of startoffset was negative or greater than the length of the |
| subject, that is, the value in length. |
| |
| PCRE_ERROR_SHORTUTF8 (-25) |
| |
| This error is returned instead of PCRE_ERROR_BADUTF8 when the subject |
| string ends with a truncated UTF-8 character and the PCRE_PARTIAL_HARD |
| option is set. Information about the failure is returned as for |
| PCRE_ERROR_BADUTF8. It is in fact sufficient to detect this case, but |
| this special error code for PCRE_PARTIAL_HARD precedes the implementa- |
| tion of returned information; it is retained for backwards compatibil- |
| ity. |
| |
| PCRE_ERROR_RECURSELOOP (-26) |
| |
| This error is returned when pcre_exec() detects a recursion loop within |
| the pattern. Specifically, it means that either the whole pattern or a |
| subpattern has been called recursively for the second time at the same |
| position in the subject string. Some simple patterns that might do this |
| are detected and faulted at compile time, but more complicated cases, |
| in particular mutual recursions between two different subpatterns, can- |
| not be detected until run time. |
| |
| PCRE_ERROR_JIT_STACKLIMIT (-27) |
| |
| This error is returned when a pattern that was successfully studied |
| using the PCRE_STUDY_JIT_COMPILE option is being matched, but the mem- |
| ory available for the just-in-time processing stack is not large |
| enough. See the pcrejit documentation for more details. |
| |
| Error numbers -16 to -20 and -22 are not used by pcre_exec(). |
| |
| Reason codes for invalid UTF-8 strings |
| |
| When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT- |
| UTF8, and the size of the output vector (ovecsize) is at least 2, the |
| offset of the start of the invalid UTF-8 character is placed in the |
| first output vector element (ovector[0]) and a reason code is placed in |
| the second element (ovector[1]). The reason codes are given names in |
| the pcre.h header file: |
| |
| PCRE_UTF8_ERR1 |
| PCRE_UTF8_ERR2 |
| PCRE_UTF8_ERR3 |
| PCRE_UTF8_ERR4 |
| PCRE_UTF8_ERR5 |
| |
| The string ends with a truncated UTF-8 character; the code specifies |
| how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8 |
| characters to be no longer than 4 bytes, the encoding scheme (origi- |
| nally defined by RFC 2279) allows for up to 6 bytes, and this is |
| checked first; hence the possibility of 4 or 5 missing bytes. |
| |
| PCRE_UTF8_ERR6 |
| PCRE_UTF8_ERR7 |
| PCRE_UTF8_ERR8 |
| PCRE_UTF8_ERR9 |
| PCRE_UTF8_ERR10 |
| |
| The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of |
| the character do not have the binary value 0b10 (that is, either the |
| most significant bit is 0, or the next bit is 1). |
| |
| PCRE_UTF8_ERR11 |
| PCRE_UTF8_ERR12 |
| |
| A character that is valid by the RFC 2279 rules is either 5 or 6 bytes |
| long; these code points are excluded by RFC 3629. |
| |
| PCRE_UTF8_ERR13 |
| |
| A 4-byte character has a value greater than 0x10fff; these code points |
| are excluded by RFC 3629. |
| |
| PCRE_UTF8_ERR14 |
| |
| A 3-byte character has a value in the range 0xd800 to 0xdfff; this |
| range of code points are reserved by RFC 3629 for use with UTF-16, and |
| so are excluded from UTF-8. |
| |
| PCRE_UTF8_ERR15 |
| PCRE_UTF8_ERR16 |
| PCRE_UTF8_ERR17 |
| PCRE_UTF8_ERR18 |
| PCRE_UTF8_ERR19 |
| |
| A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes |
| for a value that can be represented by fewer bytes, which is invalid. |
| For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor- |
| rect coding uses just one byte. |
| |
| PCRE_UTF8_ERR20 |
| |
| The two most significant bits of the first byte of a character have the |
| binary value 0b10 (that is, the most significant bit is 1 and the sec- |
| ond is 0). Such a byte can only validly occur as the second or subse- |
| quent byte of a multi-byte character. |
| |
| PCRE_UTF8_ERR21 |
| |
| The first byte of a character has the value 0xfe or 0xff. These values |
| can never occur in a valid UTF-8 string. |
| |
| |
| EXTRACTING CAPTURED SUBSTRINGS BY NUMBER |
| |
| int pcre_copy_substring(const char *subject, int *ovector, |
| int stringcount, int stringnumber, char *buffer, |
| int buffersize); |
| |
| int pcre_get_substring(const char *subject, int *ovector, |
| int stringcount, int stringnumber, |
| const char **stringptr); |
| |
| int pcre_get_substring_list(const char *subject, |
| int *ovector, int stringcount, const char ***listptr); |
| |
| Captured substrings can be accessed directly by using the offsets |
| returned by pcre_exec() in ovector. For convenience, the functions |
| pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub- |
| string_list() are provided for extracting captured substrings as new, |
| separate, zero-terminated strings. These functions identify substrings |
| by number. The next section describes functions for extracting named |
| substrings. |
| |
| A substring that contains a binary zero is correctly extracted and has |
| a further zero added on the end, but the result is not, of course, a C |
| string. However, you can process such a string by referring to the |
| length that is returned by pcre_copy_substring() and pcre_get_sub- |
| string(). Unfortunately, the interface to pcre_get_substring_list() is |
| not adequate for handling strings containing binary zeros, because the |
| end of the final string is not independently indicated. |
| |
| The first three arguments are the same for all three of these func- |
| tions: subject is the subject string that has just been successfully |
| matched, ovector is a pointer to the vector of integer offsets that was |
| passed to pcre_exec(), and stringcount is the number of substrings that |
| were captured by the match, including the substring that matched the |
| entire regular expression. This is the value returned by pcre_exec() if |
| it is greater than zero. If pcre_exec() returned zero, indicating that |
| it ran out of space in ovector, the value passed as stringcount should |
| be the number of elements in the vector divided by three. |
| |
| The functions pcre_copy_substring() and pcre_get_substring() extract a |
| single substring, whose number is given as stringnumber. A value of |
| zero extracts the substring that matched the entire pattern, whereas |
| higher values extract the captured substrings. For pcre_copy_sub- |
| string(), the string is placed in buffer, whose length is given by |
| buffersize, while for pcre_get_substring() a new block of memory is |
| obtained via pcre_malloc, and its address is returned via stringptr. |
| The yield of the function is the length of the string, not including |
| the terminating zero, or one of these error codes: |
| |
| PCRE_ERROR_NOMEMORY (-6) |
| |
| The buffer was too small for pcre_copy_substring(), or the attempt to |
| get memory failed for pcre_get_substring(). |
| |
| PCRE_ERROR_NOSUBSTRING (-7) |
| |
| There is no substring whose number is stringnumber. |
| |
| The pcre_get_substring_list() function extracts all available sub- |
| strings and builds a list of pointers to them. All this is done in a |
| single block of memory that is obtained via pcre_malloc. The address of |
| the memory block is returned via listptr, which is also the start of |
| the list of string pointers. The end of the list is marked by a NULL |
| pointer. The yield of the function is zero if all went well, or the |
| error code |
| |
| PCRE_ERROR_NOMEMORY (-6) |
| |
| if the attempt to get the memory block failed. |
| |
| When any of these functions encounter a substring that is unset, which |
| can happen when capturing subpattern number n+1 matches some part of |
| the subject, but subpattern n has not been used at all, they return an |
| empty string. This can be distinguished from a genuine zero-length sub- |
| string by inspecting the appropriate offset in ovector, which is nega- |
| tive for unset substrings. |
| |
| The two convenience functions pcre_free_substring() and pcre_free_sub- |
| string_list() can be used to free the memory returned by a previous |
| call of pcre_get_substring() or pcre_get_substring_list(), respec- |
| tively. They do nothing more than call the function pointed to by |
| pcre_free, which of course could be called directly from a C program. |
| However, PCRE is used in some situations where it is linked via a spe- |
| cial interface to another programming language that cannot use |
| pcre_free directly; it is for these cases that the functions are pro- |
| vided. |
| |
| |
| EXTRACTING CAPTURED SUBSTRINGS BY NAME |
| |
| int pcre_get_stringnumber(const pcre *code, |
| const char *name); |
| |
| int pcre_copy_named_substring(const pcre *code, |
| const char *subject, int *ovector, |
| int stringcount, const char *stringname, |
| char *buffer, int buffersize); |
| |
| int pcre_get_named_substring(const pcre *code, |
| const char *subject, int *ovector, |
| int stringcount, const char *stringname, |
| const char **stringptr); |
| |
| To extract a substring by name, you first have to find associated num- |
| ber. For example, for this pattern |
| |
| (a+)b(?<xxx>\d+)... |
| |
| the number of the subpattern called "xxx" is 2. If the name is known to |
| be unique (PCRE_DUPNAMES was not set), you can find the number from the |
| name by calling pcre_get_stringnumber(). The first argument is the com- |
| piled pattern, and the second is the name. The yield of the function is |
| the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no |
| subpattern of that name. |
| |
| Given the number, you can extract the substring directly, or use one of |
| the functions described in the previous section. For convenience, there |
| are also two functions that do the whole job. |
| |
| Most of the arguments of pcre_copy_named_substring() and |
| pcre_get_named_substring() are the same as those for the similarly |
| named functions that extract by number. As these are described in the |
| previous section, they are not re-described here. There are just two |
| differences: |
| |
| First, instead of a substring number, a substring name is given. Sec- |
| ond, there is an extra argument, given at the start, which is a pointer |
| to the compiled pattern. This is needed in order to gain access to the |
| name-to-number translation table. |
| |
| These functions call pcre_get_stringnumber(), and if it succeeds, they |
| then call pcre_copy_substring() or pcre_get_substring(), as appropri- |
| ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the |
| behaviour may not be what you want (see the next section). |
| |
| Warning: If the pattern uses the (?| feature to set up multiple subpat- |
| terns with the same number, as described in the section on duplicate |
| subpattern numbers in the pcrepattern page, you cannot use names to |
| distinguish the different subpatterns, because names are not included |
| in the compiled code. The matching process uses only numbers. For this |
| reason, the use of different names for subpatterns of the same number |
| causes an error at compile time. |
| |
| |
| DUPLICATE SUBPATTERN NAMES |
| |
| int pcre_get_stringtable_entries(const pcre *code, |
| const char *name, char **first, char **last); |
| |
| When a pattern is compiled with the PCRE_DUPNAMES option, names for |
| subpatterns are not required to be unique. (Duplicate names are always |
| allowed for subpatterns with the same number, created by using the (?| |
| feature. Indeed, if such subpatterns are named, they are required to |
| use the same names.) |
| |
| Normally, patterns with duplicate names are such that in any one match, |
| only one of the named subpatterns participates. An example is shown in |
| the pcrepattern documentation. |
| |
| When duplicates are present, pcre_copy_named_substring() and |
| pcre_get_named_substring() return the first substring corresponding to |
| the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING |
| (-7) is returned; no data is returned. The pcre_get_stringnumber() |
| function returns one of the numbers that are associated with the name, |
| but it is not defined which it is. |
| |
| If you want to get full details of all captured substrings for a given |
| name, you must use the pcre_get_stringtable_entries() function. The |
| first argument is the compiled pattern, and the second is the name. The |
| third and fourth are pointers to variables which are updated by the |
| function. After it has run, they point to the first and last entries in |
| the name-to-number table for the given name. The function itself |
| returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if |
| there are none. The format of the table is described above in the sec- |
| tion entitled Information about a pattern above. Given all the rele- |
| vant entries for the name, you can extract each of their numbers, and |
| hence the captured data, if any. |
| |
| |
| FINDING ALL POSSIBLE MATCHES |
| |
| The traditional matching function uses a similar algorithm to Perl, |
| which stops when it finds the first match, starting at a given point in |
| the subject. If you want to find all possible matches, or the longest |
| possible match, consider using the alternative matching function (see |
| below) instead. If you cannot use the alternative function, but still |
| need to find all possible matches, you can kludge it up by making use |
| of the callout facility, which is described in the pcrecallout documen- |
| tation. |
| |
| What you have to do is to insert a callout right at the end of the pat- |
| tern. When your callout function is called, extract and save the cur- |
| rent matched substring. Then return 1, which forces pcre_exec() to |
| backtrack and try other alternatives. Ultimately, when it runs out of |
| matches, pcre_exec() will yield PCRE_ERROR_NOMATCH. |
| |
| |
| MATCHING A PATTERN: THE ALTERNATIVE FUNCTION |
| |
| int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, |
| const char *subject, int length, int startoffset, |
| int options, int *ovector, int ovecsize, |
| int *workspace, int wscount); |
| |
| The function pcre_dfa_exec() is called to match a subject string |
| against a compiled pattern, using a matching algorithm that scans the |
| subject string just once, and does not backtrack. This has different |
| characteristics to the normal algorithm, and is not compatible with |
| Perl. Some of the features of PCRE patterns are not supported. Never- |
| theless, there are times when this kind of matching can be useful. For |
| a discussion of the two matching algorithms, and a list of features |
| that pcre_dfa_exec() does not support, see the pcrematching documenta- |
| tion. |
| |
| The arguments for the pcre_dfa_exec() function are the same as for |
| pcre_exec(), plus two extras. The ovector argument is used in a differ- |
| ent way, and this is described below. The other common arguments are |
| used in the same way as for pcre_exec(), so their description is not |
| repeated here. |
| |
| The two additional arguments provide workspace for the function. The |
| workspace vector should contain at least 20 elements. It is used for |
| keeping track of multiple paths through the pattern tree. More |
| workspace will be needed for patterns and subjects where there are a |
| lot of potential matches. |
| |
| Here is an example of a simple call to pcre_dfa_exec(): |
| |
| int rc; |
| int ovector[10]; |
| int wspace[20]; |
| rc = pcre_dfa_exec( |
| re, /* result of pcre_compile() */ |
| NULL, /* we didn't study the pattern */ |
| "some string", /* the subject string */ |
| 11, /* the length of the subject string */ |
| 0, /* start at offset 0 in the subject */ |
| 0, /* default options */ |
| ovector, /* vector of integers for substring information */ |
| 10, /* number of elements (NOT size in bytes) */ |
| wspace, /* working space vector */ |
| 20); /* number of elements (NOT size in bytes) */ |
| |
| Option bits for pcre_dfa_exec() |
| |
| The unused bits of the options argument for pcre_dfa_exec() must be |
| zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW- |
| LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, |
| PCRE_NOTEMPTY_ATSTART, PCRE_NO_UTF8_CHECK, PCRE_BSR_ANYCRLF, |
| PCRE_BSR_UNICODE, PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD, PCRE_PAR- |
| TIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last |
| four of these are exactly the same as for pcre_exec(), so their |
| description is not repeated here. |
| |
| PCRE_PARTIAL_HARD |
| PCRE_PARTIAL_SOFT |
| |
| These have the same general effect as they do for pcre_exec(), but the |
| details are slightly different. When PCRE_PARTIAL_HARD is set for |
| pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub- |
| ject is reached and there is still at least one matching possibility |
| that requires additional characters. This happens even if some complete |
| matches have also been found. When PCRE_PARTIAL_SOFT is set, the return |
| code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end |
| of the subject is reached, there have been no complete matches, but |
| there is still at least one matching possibility. The portion of the |
| string that was inspected when the longest partial match was found is |
| set as the first matching string in both cases. There is a more |
| detailed discussion of partial and multi-segment matching, with exam- |
| ples, in the pcrepartial documentation. |
| |
| PCRE_DFA_SHORTEST |
| |
| Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to |
| stop as soon as it has found one match. Because of the way the alterna- |
| tive algorithm works, this is necessarily the shortest possible match |
| at the first possible matching point in the subject string. |
| |
| PCRE_DFA_RESTART |
| |
| When pcre_dfa_exec() returns a partial match, it is possible to call it |
| again, with additional subject characters, and have it continue with |
| the same match. The PCRE_DFA_RESTART option requests this action; when |
| it is set, the workspace and wscount options must reference the same |
| vector as before because data about the match so far is left in them |
| after a partial match. There is more discussion of this facility in the |
| pcrepartial documentation. |
| |
| Successful returns from pcre_dfa_exec() |
| |
| When pcre_dfa_exec() succeeds, it may have matched more than one sub- |
| string in the subject. Note, however, that all the matches from one run |
| of the function start at the same point in the subject. The shorter |
| matches are all initial substrings of the longer matches. For example, |
| if the pattern |
| |
| <.*> |
| |
| is matched against the string |
| |
| This is <something> <something else> <something further> no more |
| |
| the three matched strings are |
| |
| <something> |
| <something> <something else> |
| <something> <something else> <something further> |
| |
| On success, the yield of the function is a number greater than zero, |
| which is the number of matched substrings. The substrings themselves |
| are returned in ovector. Each string uses two elements; the first is |
| the offset to the start, and the second is the offset to the end. In |
| fact, all the strings have the same start offset. (Space could have |
| been saved by giving this only once, but it was decided to retain some |
| compatibility with the way pcre_exec() returns data, even though the |
| meaning of the strings is different.) |
| |
| The strings are returned in reverse order of length; that is, the long- |
| est matching string is given first. If there were too many matches to |
| fit into ovector, the yield of the function is zero, and the vector is |
| filled with the longest matches. Unlike pcre_exec(), pcre_dfa_exec() |
| can use the entire ovector for returning matched strings. |
| |
| Error returns from pcre_dfa_exec() |
| |
| The pcre_dfa_exec() function returns a negative number when it fails. |
| Many of the errors are the same as for pcre_exec(), and these are |
| described above. There are in addition the following errors that are |
| specific to pcre_dfa_exec(): |
| |
| PCRE_ERROR_DFA_UITEM (-16) |
| |
| This return is given if pcre_dfa_exec() encounters an item in the pat- |
| tern that it does not support, for instance, the use of \C or a back |
| reference. |
| |
| PCRE_ERROR_DFA_UCOND (-17) |
| |
| This return is given if pcre_dfa_exec() encounters a condition item |
| that uses a back reference for the condition, or a test for recursion |
| in a specific group. These are not supported. |
| |
| PCRE_ERROR_DFA_UMLIMIT (-18) |
| |
| This return is given if pcre_dfa_exec() is called with an extra block |
| that contains a setting of the match_limit or match_limit_recursion |
| fields. This is not supported (these fields are meaningless for DFA |
| matching). |
| |
| PCRE_ERROR_DFA_WSSIZE (-19) |
| |
| This return is given if pcre_dfa_exec() runs out of space in the |
| workspace vector. |
| |
| PCRE_ERROR_DFA_RECURSE (-20) |
| |
| When a recursive subpattern is processed, the matching function calls |
| itself recursively, using private vectors for ovector and workspace. |
| This error is given if the output vector is not large enough. This |
| should be extremely rare, as a vector of size 1000 is used. |
| |
| |
| SEE ALSO |
| |
| pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar- |
| tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3). |
| |
| |
| AUTHOR |
| |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| |
| REVISION |
| |
| Last updated: 02 December 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| |
| |
| PCRECALLOUT(3) PCRECALLOUT(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions |
| |
| |
| PCRE CALLOUTS |
| |
| int (*pcre_callout)(pcre_callout_block *); |
| |
| PCRE provides a feature called "callout", which is a means of temporar- |
| ily passing control to the caller of PCRE in the middle of pattern |
| matching. The caller of PCRE provides an external function by putting |
| its entry point in the global variable pcre_callout. By default, this |
| variable contains NULL, which disables all calling out. |
| |
| Within a regular expression, (?C) indicates the points at which the |
| external function is to be called. Different callout points can be |
| identified by putting a number less than 256 after the letter C. The |
| default value is zero. For example, this pattern has two callout |
| points: |
| |
| (?C1)abc(?C2)def |
| |
| If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() or |
| pcre_compile2() is called, PCRE automatically inserts callouts, all |
| with number 255, before each item in the pattern. For example, if |
| PCRE_AUTO_CALLOUT is used with the pattern |
| |
| A(\d{2}|--) |
| |
| it is processed as if it were |
| |
| (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255) |
| |
| Notice that there is a callout before and after each parenthesis and |
| alternation bar. Automatic callouts can be used for tracking the |
| progress of pattern matching. The pcretest command has an option that |
| sets automatic callouts; when it is used, the output indicates how the |
| pattern is matched. This is useful information when you are trying to |
| optimize the performance of a particular pattern. |
| |
| The use of callouts in a pattern makes it ineligible for optimization |
| by the just-in-time compiler. Studying such a pattern with the |
| PCRE_STUDY_JIT_COMPILE option always fails. |
| |
| |
| MISSING CALLOUTS |
| |
| You should be aware that, because of optimizations in the way PCRE |
| matches patterns by default, callouts sometimes do not happen. For |
| example, if the pattern is |
| |
| ab(?C4)cd |
| |
| PCRE knows that any matching string must contain the letter "d". If the |
| subject string is "abyz", the lack of "d" means that matching doesn't |
| ever start, and the callout is never reached. However, with "abyd", |
| though the result is still no match, the callout is obeyed. |
| |
| If the pattern is studied, PCRE knows the minimum length of a matching |
| string, and will immediately give a "no match" return without actually |
| running a match if the subject is not long enough, or, for unanchored |
| patterns, if it has been scanned far enough. |
| |
| You can disable these optimizations by passing the PCRE_NO_START_OPTI- |
| MIZE option to pcre_compile(), pcre_exec(), or pcre_dfa_exec(), or by |
| starting the pattern with (*NO_START_OPT). This slows down the matching |
| process, but does ensure that callouts such as the example above are |
| obeyed. |
| |
| |
| THE CALLOUT INTERFACE |
| |
| During matching, when PCRE reaches a callout point, the external func- |
| tion defined by pcre_callout is called (if it is set). This applies to |
| both the pcre_exec() and the pcre_dfa_exec() matching functions. The |
| only argument to the callout function is a pointer to a pcre_callout |
| block. This structure contains the following fields: |
| |
| int version; |
| int callout_number; |
| int *offset_vector; |
| const char *subject; |
| int subject_length; |
| int start_match; |
| int current_position; |
| int capture_top; |
| int capture_last; |
| void *callout_data; |
| int pattern_position; |
| int next_item_length; |
| const unsigned char *mark; |
| |
| The version field is an integer containing the version number of the |
| block format. The initial version was 0; the current version is 2. The |
| version number will change again in future if additional fields are |
| added, but the intention is never to remove any of the existing fields. |
| |
| The callout_number field contains the number of the callout, as com- |
| piled into the pattern (that is, the number after ?C for manual call- |
| outs, and 255 for automatically generated callouts). |
| |
| The offset_vector field is a pointer to the vector of offsets that was |
| passed by the caller to pcre_exec() or pcre_dfa_exec(). When |
| pcre_exec() is used, the contents can be inspected in order to extract |
| substrings that have been matched so far, in the same way as for |
| extracting substrings after a match has completed. For pcre_dfa_exec() |
| this field is not useful. |
| |
| The subject and subject_length fields contain copies of the values that |
| were passed to pcre_exec(). |
| |
| The start_match field normally contains the offset within the subject |
| at which the current match attempt started. However, if the escape |
| sequence \K has been encountered, this value is changed to reflect the |
| modified starting point. If the pattern is not anchored, the callout |
| function may be called several times from the same point in the pattern |
| for different starting points in the subject. |
| |
| The current_position field contains the offset within the subject of |
| the current match pointer. |
| |
| When the pcre_exec() function is used, the capture_top field contains |
| one more than the number of the highest numbered captured substring so |
| far. If no substrings have been captured, the value of capture_top is |
| one. This is always the case when pcre_dfa_exec() is used, because it |
| does not support captured substrings. |
| |
| The capture_last field contains the number of the most recently cap- |
| tured substring. If no substrings have been captured, its value is -1. |
| This is always the case when pcre_dfa_exec() is used. |
| |
| The callout_data field contains a value that is passed to pcre_exec() |
| or pcre_dfa_exec() specifically so that it can be passed back in call- |
| outs. It is passed in the pcre_callout field of the pcre_extra data |
| structure. If no such data was passed, the value of callout_data in a |
| pcre_callout block is NULL. There is a description of the pcre_extra |
| structure in the pcreapi documentation. |
| |
| The pattern_position field is present from version 1 of the pcre_call- |
| out structure. It contains the offset to the next item to be matched in |
| the pattern string. |
| |
| The next_item_length field is present from version 1 of the pcre_call- |
| out structure. It contains the length of the next item to be matched in |
| the pattern string. When the callout immediately precedes an alterna- |
| tion bar, a closing parenthesis, or the end of the pattern, the length |
| is zero. When the callout precedes an opening parenthesis, the length |
| is that of the entire subpattern. |
| |
| The pattern_position and next_item_length fields are intended to help |
| in distinguishing between different automatic callouts, which all have |
| the same callout number. However, they are set for all callouts. |
| |
| The mark field is present from version 2 of the pcre_callout structure. |
| In callouts from pcre_exec() it contains a pointer to the zero-termi- |
| nated name of the most recently passed (*MARK), (*PRUNE), or (*THEN) |
| item in the match, or NULL if no such items have been passed. Instances |
| of (*PRUNE) or (*THEN) without a name do not obliterate a previous |
| (*MARK). In callouts from pcre_dfa_exec() this field always contains |
| NULL. |
| |
| |
| RETURN VALUES |
| |
| The external callout function returns an integer to PCRE. If the value |
| is zero, matching proceeds as normal. If the value is greater than |
| zero, matching fails at the current point, but the testing of other |
| matching possibilities goes ahead, just as if a lookahead assertion had |
| failed. If the value is less than zero, the match is abandoned, and |
| pcre_exec() or pcre_dfa_exec() returns the negative value. |
| |
| Negative values should normally be chosen from the set of |
| PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan- |
| dard "no match" failure. The error number PCRE_ERROR_CALLOUT is |
| reserved for use by callout functions; it will never be used by PCRE |
| itself. |
| |
| |
| AUTHOR |
| |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| |
| REVISION |
| |
| Last updated: 30 November 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| |
| |
| PCRECOMPAT(3) PCRECOMPAT(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions |
| |
| |
| DIFFERENCES BETWEEN PCRE AND PERL |
| |
| This document describes the differences in the ways that PCRE and Perl |
| handle regular expressions. The differences described here are with |
| respect to Perl versions 5.10 and above. |
| |
| 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details |
| of what it does have are given in the pcreunicode page. |
| |
| 2. PCRE allows repeat quantifiers only on parenthesized assertions, but |
| they do not mean what you might think. For example, (?!a){3} does not |
| assert that the next three characters are not "a". It just asserts that |
| the next character is not "a" three times (in principle: PCRE optimizes |
| this to run the assertion just once). Perl allows repeat quantifiers on |
| other assertions such as \b, but these do not seem to have any use. |
| |
| 3. Capturing subpatterns that occur inside negative lookahead asser- |
| tions are counted, but their entries in the offsets vector are never |
| set. Perl sets its numerical variables from any such patterns that are |
| matched before the assertion fails to match something (thereby succeed- |
| ing), but only if the negative lookahead assertion contains just one |
| branch. |
| |
| 4. Though binary zero characters are supported in the subject string, |
| they are not allowed in a pattern string because it is passed as a nor- |
| mal C string, terminated by zero. The escape sequence \0 can be used in |
| the pattern to represent a binary zero. |
| |
| 5. The following Perl escape sequences are not supported: \l, \u, \L, |
| \U, and \N when followed by a character name or Unicode value. (\N on |
| its own, matching a non-newline character, is supported.) In fact these |
| are implemented by Perl's general string-handling and are not part of |
| its pattern matching engine. If any of these are encountered by PCRE, |
| an error is generated by default. However, if the PCRE_JAVASCRIPT_COM- |
| PAT option is set, \U and \u are interpreted as JavaScript interprets |
| them. |
| |
| 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE |
| is built with Unicode character property support. The properties that |
| can be tested with \p and \P are limited to the general category prop- |
| erties such as Lu and Nd, script names such as Greek or Han, and the |
| derived properties Any and L&. PCRE does support the Cs (surrogate) |
| property, which Perl does not; the Perl documentation says "Because |
| Perl hides the need for the user to understand the internal representa- |
| tion of Unicode characters, there is no need to implement the somewhat |
| messy concept of surrogates." |
| |
| 7. PCRE implements a simpler version of \X than Perl, which changed to |
| make \X match what Unicode calls an "extended grapheme cluster". This |
| is more complicated than an extended Unicode sequence, which is what |
| PCRE matches. |
| |
| 8. PCRE does support the \Q...\E escape for quoting substrings. Charac- |
| ters in between are treated as literals. This is slightly different |
| from Perl in that $ and @ are also handled as literals inside the |
| quotes. In Perl, they cause variable interpolation (but of course PCRE |
| does not have variables). Note the following examples: |
| |
| Pattern PCRE matches Perl matches |
| |
| \Qabc$xyz\E abc$xyz abc followed by the |
| contents of $xyz |
| \Qabc\$xyz\E abc\$xyz abc\$xyz |
| \Qabc\E\$\Qxyz\E abc$xyz abc$xyz |
| |
| The \Q...\E sequence is recognized both inside and outside character |
| classes. |
| |
| 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code}) |
| constructions. However, there is support for recursive patterns. This |
| is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE |
| "callout" feature allows an external function to be called during pat- |
| tern matching. See the pcrecallout documentation for details. |
| |
| 10. Subpatterns that are called as subroutines (whether or not recur- |
| sively) are always treated as atomic groups in PCRE. This is like |
| Python, but unlike Perl. Captured values that are set outside a sub- |
| routine call can be reference from inside in PCRE, but not in Perl. |
| There is a discussion that explains these differences in more detail in |
| the section on recursion differences from Perl in the pcrepattern page. |
| |
| 11. If (*THEN) is present in a group that is called as a subroutine, |
| its action is limited to that group, even if the group does not contain |
| any | characters. |
| |
| 12. There are some differences that are concerned with the settings of |
| captured strings when part of a pattern is repeated. For example, |
| matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 |
| unset, but in PCRE it is set to "b". |
| |
| 13. PCRE's handling of duplicate subpattern numbers and duplicate sub- |
| pattern names is not as general as Perl's. This is a consequence of the |
| fact the PCRE works internally just with numbers, using an external ta- |
| ble to translate between numbers and names. In particular, a pattern |
| such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have |
| the same number but different names, is not supported, and causes an |
| error at compile time. If it were allowed, it would not be possible to |
| distinguish which parentheses matched, because both names map to cap- |
| turing subpattern number 1. To avoid this confusing situation, an error |
| is given at compile time. |
| |
| 14. Perl recognizes comments in some places that PCRE does not, for |
| example, between the ( and ? at the start of a subpattern. If the /x |
| modifier is set, Perl allows whitespace between ( and ? but PCRE never |
| does, even if the PCRE_EXTENDED option is set. |
| |
| 15. PCRE provides some extensions to the Perl regular expression facil- |
| ities. Perl 5.10 includes new features that are not in earlier ver- |
| sions of Perl, some of which (such as named parentheses) have been in |
| PCRE for some time. This list is with respect to Perl 5.10: |
| |
| (a) Although lookbehind assertions in PCRE must match fixed length |
| strings, each alternative branch of a lookbehind assertion can match a |
| different length of string. Perl requires them all to have the same |
| length. |
| |
| (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ |
| meta-character matches only at the very end of the string. |
| |
| (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe- |
| cial meaning is faulted. Otherwise, like Perl, the backslash is quietly |
| ignored. (Perl can be made to issue a warning.) |
| |
| (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti- |
| fiers is inverted, that is, by default they are not greedy, but if fol- |
| lowed by a question mark they are. |
| |
| (e) PCRE_ANCHORED can be used at matching time to force a pattern to be |
| tried only at the first matching position in the subject string. |
| |
| (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, |
| and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva- |
| lents. |
| |
| (g) The \R escape sequence can be restricted to match only CR, LF, or |
| CRLF by the PCRE_BSR_ANYCRLF option. |
| |
| (h) The callout facility is PCRE-specific. |
| |
| (i) The partial matching facility is PCRE-specific. |
| |
| (j) Patterns compiled by PCRE can be saved and re-used at a later time, |
| even on different hosts that have the other endianness. However, this |
| does not apply to optimized data created by the just-in-time compiler. |
| |
| (k) The alternative matching function (pcre_dfa_exec()) matches in a |
| different way and is not Perl-compatible. |
| |
| (l) PCRE recognizes some special sequences such as (*CR) at the start |
| of a pattern that set overall options that cannot be changed within the |
| pattern. |
| |
| |
| AUTHOR |
| |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| |
| REVISION |
| |
| Last updated: 14 November 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| |
| |
| PCREPATTERN(3) PCREPATTERN(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions |
| |
| |
| PCRE REGULAR EXPRESSION DETAILS |
| |
| The syntax and semantics of the regular expressions that are supported |
| by PCRE are described in detail below. There is a quick-reference syn- |
| tax summary in the pcresyntax page. PCRE tries to match Perl syntax and |
| semantics as closely as it can. PCRE also supports some alternative |
| regular expression syntax (which does not conflict with the Perl syn- |
| tax) in order to provide some compatibility with regular expressions in |
| Python, .NET, and Oniguruma. |
| |
| Perl's regular expressions are described in its own documentation, and |
| regular expressions in general are covered in a number of books, some |
| of which have copious examples. Jeffrey Friedl's "Mastering Regular |
| Expressions", published by O'Reilly, covers regular expressions in |
| great detail. This description of PCRE's regular expressions is |
| intended as reference material. |
| |
| The original operation of PCRE was on strings of one-byte characters. |
| However, there is now also support for UTF-8 character strings. To use |
| this, PCRE must be built to include UTF-8 support, and you must call |
| pcre_compile() or pcre_compile2() with the PCRE_UTF8 option. There is |
| also a special sequence that can be given at the start of a pattern: |
| |
| (*UTF8) |
| |
| Starting a pattern with this sequence is equivalent to setting the |
| PCRE_UTF8 option. This feature is not Perl-compatible. How setting |
| UTF-8 mode affects pattern matching is mentioned in several places |
| below. There is also a summary of UTF-8 features in the pcreunicode |
| page. |
| |
| Another special sequence that may appear at the start of a pattern or |
| in combination with (*UTF8) is: |
| |
| (*UCP) |
| |
| This has the same effect as setting the PCRE_UCP option: it causes |
| sequences such as \d and \w to use Unicode properties to determine |
| character types, instead of recognizing only characters with codes less |
| than 128 via a lookup table. |
| |
| If a pattern starts with (*NO_START_OPT), it has the same effect as |
| setting the PCRE_NO_START_OPTIMIZE option either at compile or matching |
| time. There are also some more of these special sequences that are con- |
| cerned with the handling of newlines; they are described below. |
| |
| The remainder of this document discusses the patterns that are sup- |
| ported by PCRE when its main matching function, pcre_exec(), is used. |
| From release 6.0, PCRE offers a second matching function, |
| pcre_dfa_exec(), which matches using a different algorithm that is not |
| Perl-compatible. Some of the features discussed below are not available |
| when pcre_dfa_exec() is used. The advantages and disadvantages of the |
| alternative function, and how it differs from the normal function, are |
| discussed in the pcrematching page. |
| |
| |
| NEWLINE CONVENTIONS |
| |
| PCRE supports five different conventions for indicating line breaks in |
| strings: a single CR (carriage return) character, a single LF (line- |
| feed) character, the two-character sequence CRLF, any of the three pre- |
| ceding, or any Unicode newline sequence. The pcreapi page has further |
| discussion about newlines, and shows how to set the newline convention |
| in the options arguments for the compiling and matching functions. |
| |
| It is also possible to specify a newline convention by starting a pat- |
| tern string with one of the following five sequences: |
| |
| (*CR) carriage return |
| (*LF) linefeed |
| (*CRLF) carriage return, followed by linefeed |
| (*ANYCRLF) any of the three above |
| (*ANY) all Unicode newline sequences |
| |
| These override the default and the options given to pcre_compile() or |
| pcre_compile2(). For example, on a Unix system where LF is the default |
| newline sequence, the pattern |
| |
| (*CR)a.b |
| |
| changes the convention to CR. That pattern matches "a\nb" because LF is |
| no longer a newline. Note that these special settings, which are not |
| Perl-compatible, are recognized only at the very start of a pattern, |
| and that they must be in upper case. If more than one of them is |
| present, the last one is used. |
| |
| The newline convention affects the interpretation of the dot metachar- |
| acter when PCRE_DOTALL is not set, and also the behaviour of \N. How- |
| ever, it does not affect what the \R escape sequence matches. By |
| default, this is any Unicode newline sequence, for Perl compatibility. |
| However, this can be changed; see the description of \R in the section |
| entitled "Newline sequences" below. A change of \R setting can be com- |
| bined with a change of newline convention. |
| |
| |
| CHARACTERS AND METACHARACTERS |
| |
| A regular expression is a pattern that is matched against a subject |
| string from left to right. Most characters stand for themselves in a |
| pattern, and match the corresponding characters in the subject. As a |
| trivial example, the pattern |
| |
| The quick brown fox |
| |
| matches a portion of a subject string that is identical to itself. When |
| caseless matching is specified (the PCRE_CASELESS option), letters are |
| matched independently of case. In UTF-8 mode, PCRE always understands |
| the concept of case for characters whose values are less than 128, so |
| caseless matching is always possible. For characters with higher val- |
| ues, the concept of case is supported if PCRE is compiled with Unicode |
| property support, but not otherwise. If you want to use caseless |
| matching for characters 128 and above, you must ensure that PCRE is |
| compiled with Unicode property support as well as with UTF-8 support. |
| |
| The power of regular expressions comes from the ability to include |
| alternatives and repetitions in the pattern. These are encoded in the |
| pattern by the use of metacharacters, which do not stand for themselves |
| but instead are interpreted in some special way. |
| |
| There are two different sets of metacharacters: those that are recog- |
| nized anywhere in the pattern except within square brackets, and those |
| that are recognized within square brackets. Outside square brackets, |
| the metacharacters are as follows: |
| |
| \ general escape character with several uses |
| ^ assert start of string (or line, in multiline mode) |
| $ assert end of string (or line, in multiline mode) |
| . match any character except newline (by default) |
| [ start character class definition |
| | start of alternative branch |
| ( start subpattern |
| ) end subpattern |
| ? extends the meaning of ( |
| also 0 or 1 quantifier |
| also quantifier minimizer |
| * 0 or more quantifier |
| + 1 or more quantifier |
| also "possessive quantifier" |
| { start min/max quantifier |
| |
| Part of a pattern that is in square brackets is called a "character |
| class". In a character class the only metacharacters are: |
| |
| \ general escape character |
| ^ negate the class, but only if the first character |
| - indicates character range |
| [ POSIX character class (only if followed by POSIX |
| syntax) |
| ] terminates the character class |
| |
| The following sections describe the use of each of the metacharacters. |
| |
| |
| BACKSLASH |
| |
| The backslash character has several uses. Firstly, if it is followed by |
| a character that is not a number or a letter, it takes away any special |
| meaning that character may have. This use of backslash as an escape |
| character applies both inside and outside character classes. |
| |
| For example, if you want to match a * character, you write \* in the |
| pattern. This escaping action applies whether or not the following |
| character would otherwise be interpreted as a metacharacter, so it is |
| always safe to precede a non-alphanumeric with backslash to specify |
| that it stands for itself. In particular, if you want to match a back- |
| slash, you write \\. |
| |
| In UTF-8 mode, only ASCII numbers and letters have any special meaning |
| after a backslash. All other characters (in particular, those whose |
| codepoints are greater than 127) are treated as literals. |
| |
| If a pattern is compiled with the PCRE_EXTENDED option, whitespace in |
| the pattern (other than in a character class) and characters between a |
| # outside a character class and the next newline are ignored. An escap- |
| ing backslash can be used to include a whitespace or # character as |
| part of the pattern. |
| |
| If you want to remove the special meaning from a sequence of charac- |
| ters, you can do so by putting them between \Q and \E. This is differ- |
| ent from Perl in that $ and @ are handled as literals in \Q...\E |
| sequences in PCRE, whereas in Perl, $ and @ cause variable interpola- |
| tion. Note the following examples: |
| |
| Pattern PCRE matches Perl matches |
| |
| \Qabc$xyz\E abc$xyz abc followed by the |
| contents of $xyz |
| \Qabc\$xyz\E abc\$xyz abc\$xyz |
| \Qabc\E\$\Qxyz\E abc$xyz abc$xyz |
| |
| The \Q...\E sequence is recognized both inside and outside character |
| classes. An isolated \E that is not preceded by \Q is ignored. If \Q |
| is not followed by \E later in the pattern, the literal interpretation |
| continues to the end of the pattern (that is, \E is assumed at the |
| end). If the isolated \Q is inside a character class, this causes an |
| error, because the character class is not terminated. |
| |
| Non-printing characters |
| |
| A second use of backslash provides a way of encoding non-printing char- |
| acters in patterns in a visible manner. There is no restriction on the |
| appearance of non-printing characters, apart from the binary zero that |
| terminates a pattern, but when a pattern is being prepared by text |
| editing, it is often easier to use one of the following escape |
| sequences than the binary character it represents: |
| |
| \a alarm, that is, the BEL character (hex 07) |
| \cx "control-x", where x is any ASCII character |
| \e escape (hex 1B) |
| \f formfeed (hex 0C) |
| \n linefeed (hex 0A) |
| \r carriage return (hex 0D) |
| \t tab (hex 09) |
| \ddd character with octal code ddd, or back reference |
| \xhh character with hex code hh |
| \x{hhh..} character with hex code hhh.. (non-JavaScript mode) |
| \uhhhh character with hex code hhhh (JavaScript mode only) |
| |
| The precise effect of \cx is as follows: if x is a lower case letter, |
| it is converted to upper case. Then bit 6 of the character (hex 40) is |
| inverted. Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ |
| is 7B), while \c; becomes hex 7B (; is 3B). If the byte following \c |
| has a value greater than 127, a compile-time error occurs. This locks |
| out non-ASCII characters in both byte mode and UTF-8 mode. (When PCRE |
| is compiled in EBCDIC mode, all byte values are valid. A lower case |
| letter is converted to upper case, and then the 0xc0 bits are flipped.) |
| |
| By default, after \x, from zero to two hexadecimal digits are read |
| (letters can be in upper or lower case). Any number of hexadecimal dig- |
| its may appear between \x{ and }, but the value of the character code |
| must be less than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 |
| mode. That is, the maximum value in hexadecimal is 7FFFFFFF. Note that |
| this is bigger than the largest Unicode code point, which is 10FFFF. |
| |
| If characters other than hexadecimal digits appear between \x{ and }, |
| or if there is no terminating }, this form of escape is not recognized. |
| Instead, the initial \x will be interpreted as a basic hexadecimal |
| escape, with no following digits, giving a character whose value is |
| zero. |
| |
| If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x |
| is as just described only when it is followed by two hexadecimal dig- |
| its. Otherwise, it matches a literal "x" character. In JavaScript |
| mode, support for code points greater than 256 is provided by \u, which |
| must be followed by four hexadecimal digits; otherwise it matches a |
| literal "u" character. |
| |
| Characters whose value is less than 256 can be defined by either of the |
| two syntaxes for \x (or by \u in JavaScript mode). There is no differ- |
| ence in the way they are handled. For example, \xdc is exactly the same |
| as \x{dc} (or \u00dc in JavaScript mode). |
| |
| After \0 up to two further octal digits are read. If there are fewer |
| than two digits, just those that are present are used. Thus the |
| sequence \0\x\07 specifies two binary zeros followed by a BEL character |
| (code value 7). Make sure you supply two digits after the initial zero |
| if the pattern character that follows is itself an octal digit. |
| |
| The handling of a backslash followed by a digit other than 0 is compli- |
| cated. Outside a character class, PCRE reads it and any following dig- |
| its as a decimal number. If the number is less than 10, or if there |
| have been at least that many previous capturing left parentheses in the |
| expression, the entire sequence is taken as a back reference. A |
| description of how this works is given later, following the discussion |
| of parenthesized subpatterns. |
| |
| Inside a character class, or if the decimal number is greater than 9 |
| and there have not been that many capturing subpatterns, PCRE re-reads |
| up to three octal digits following the backslash, and uses them to gen- |
| erate a data character. Any subsequent digits stand for themselves. In |
| non-UTF-8 mode, the value of a character specified in octal must be |
| less than \400. In UTF-8 mode, values up to \777 are permitted. For |
| example: |
| |
| \040 is another way of writing a space |
| \40 is the same, provided there are fewer than 40 |
| previous capturing subpatterns |
| \7 is always a back reference |
| \11 might be a back reference, or another way of |
| writing a tab |
| \011 is always a tab |
| \0113 is a tab followed by the character "3" |
| \113 might be a back reference, otherwise the |
| character with octal code 113 |
| \377 might be a back reference, otherwise |
| the byte consisting entirely of 1 bits |
| \81 is either a back reference, or a binary zero |
| followed by the two characters "8" and "1" |
| |
| Note that octal values of 100 or greater must not be introduced by a |
| leading zero, because no more than three octal digits are ever read. |
| |
| All the sequences that define a single character value can be used both |
| inside and outside character classes. In addition, inside a character |
| class, \b is interpreted as the backspace character (hex 08). |
| |
| \N is not allowed in a character class. \B, \R, and \X are not special |
| inside a character class. Like other unrecognized escape sequences, |
| they are treated as the literal characters "B", "R", and "X" by |
| default, but cause an error if the PCRE_EXTRA option is set. Outside a |
| character class, these sequences have different meanings. |
| |
| Unsupported escape sequences |
| |
| In Perl, the sequences \l, \L, \u, and \U are recognized by its string |
| handler and used to modify the case of following characters. By |
| default, PCRE does not support these escape sequences. However, if the |
| PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and |
| \u can be used to define a character by code point, as described in the |
| previous section. |
| |
| Absolute and relative back references |
| |
| The sequence \g followed by an unsigned or a negative number, option- |
| ally enclosed in braces, is an absolute or relative back reference. A |
| named back reference can be coded as \g{name}. Back references are dis- |
| cussed later, following the discussion of parenthesized subpatterns. |
| |
| Absolute and relative subroutine calls |
| |
| For compatibility with Oniguruma, the non-Perl syntax \g followed by a |
| name or a number enclosed either in angle brackets or single quotes, is |
| an alternative syntax for referencing a subpattern as a "subroutine". |
| Details are discussed later. Note that \g{...} (Perl syntax) and |
| \g<...> (Oniguruma syntax) are not synonymous. The former is a back |
| reference; the latter is a subroutine call. |
| |
| Generic character types |
| |
| Another use of backslash is for specifying generic character types: |
| |
| \d any decimal digit |
| \D any character that is not a decimal digit |
| \h any horizontal whitespace character |
| \H any character that is not a horizontal whitespace character |
| \s any whitespace character |
| \S any character that is not a whitespace character |
| \v any vertical whitespace character |
| \V any character that is not a vertical whitespace character |
| \w any "word" character |
| \W any "non-word" character |
| |
| There is also the single sequence \N, which matches a non-newline char- |
| acter. This is the same as the "." metacharacter when PCRE_DOTALL is |
| not set. Perl also uses \N to match characters by name; PCRE does not |
| support this. |
| |
| Each pair of lower and upper case escape sequences partitions the com- |
| plete set of characters into two disjoint sets. Any given character |
| matches one, and only one, of each pair. The sequences can appear both |
| inside and outside character classes. They each match one character of |
| the appropriate type. If the current matching point is at the end of |
| the subject string, all of them fail, because there is no character to |
| match. |
| |
| For compatibility with Perl, \s does not match the VT character (code |
| 11). This makes it different from the the POSIX "space" class. The \s |
| characters are HT (9), LF (10), FF (12), CR (13), and space (32). If |
| "use locale;" is included in a Perl script, \s may match the VT charac- |
| ter. In PCRE, it never does. |
| |
| A "word" character is an underscore or any character that is a letter |
| or digit. By default, the definition of letters and digits is con- |
| trolled by PCRE's low-valued character tables, and may vary if locale- |
| specific matching is taking place (see "Locale support" in the pcreapi |
| page). For example, in a French locale such as "fr_FR" in Unix-like |
| systems, or "french" in Windows, some character codes greater than 128 |
| are used for accented letters, and these are then matched by \w. The |
| use of locales with Unicode is discouraged. |
| |
| By default, in UTF-8 mode, characters with values greater than 128 |
| never match \d, \s, or \w, and always match \D, \S, and \W. These |
| sequences retain their original meanings from before UTF-8 support was |
| available, mainly for efficiency reasons. However, if PCRE is compiled |
| with Unicode property support, and the PCRE_UCP option is set, the be- |
| haviour is changed so that Unicode properties are used to determine |
| character types, as follows: |
| |
| \d any character that \p{Nd} matches (decimal digit) |
| \s any character that \p{Z} matches, plus HT, LF, FF, CR |
| \w any character that \p{L} or \p{N} matches, plus underscore |
| |
| The upper case escapes match the inverse sets of characters. Note that |
| \d matches only decimal digits, whereas \w matches any Unicode digit, |
| as well as any Unicode letter, and underscore. Note also that PCRE_UCP |
| affects \b, and \B because they are defined in terms of \w and \W. |
| Matching these sequences is noticeably slower when PCRE_UCP is set. |
| |
| The sequences \h, \H, \v, and \V are features that were added to Perl |
| at release 5.10. In contrast to the other sequences, which match only |
| ASCII characters by default, these always match certain high-valued |
| codepoints in UTF-8 mode, whether or not PCRE_UCP is set. The horizon- |
| tal space characters are: |
| |
| U+0009 Horizontal tab |
| U+0020 Space |
| U+00A0 Non-break space |
| U+1680 Ogham space mark |
| U+180E Mongolian vowel separator |
| U+2000 En quad |
| U+2001 Em quad |
| U+2002 En space |
| U+2003 Em space |
| U+2004 Three-per-em space |
| U+2005 Four-per-em space |
| U+2006 Six-per-em space |
| U+2007 Figure space |
| U+2008 Punctuation space |
| U+2009 Thin space |
| U+200A Hair space |
| U+202F Narrow no-break space |
| U+205F Medium mathematical space |
| U+3000 Ideographic space |
| |
| The vertical space characters are: |
| |
| U+000A Linefeed |
| U+000B Vertical tab |
| U+000C Formfeed |
| U+000D Carriage return |
| U+0085 Next line |
| U+2028 Line separator |
| U+2029 Paragraph separator |
| |
| Newline sequences |
| |
| Outside a character class, by default, the escape sequence \R matches |
| any Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the |
| following: |
| |
| (?>\r\n|\n|\x0b|\f|\r|\x85) |
| |
| This is an example of an "atomic group", details of which are given |
| below. This particular group matches either the two-character sequence |
| CR followed by LF, or one of the single characters LF (linefeed, |
| U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage |
| return, U+000D), or NEL (next line, U+0085). The two-character sequence |
| is treated as a single unit that cannot be split. |
| |
| In UTF-8 mode, two additional characters whose codepoints are greater |
| than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa- |
| rator, U+2029). Unicode character property support is not needed for |
| these characters to be recognized. |
| |
| It is possible to restrict \R to match only CR, LF, or CRLF (instead of |
| the complete set of Unicode line endings) by setting the option |
| PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched. |
| (BSR is an abbrevation for "backslash R".) This can be made the default |
| when PCRE is built; if this is the case, the other behaviour can be |
| requested via the PCRE_BSR_UNICODE option. It is also possible to |
| specify these settings by starting a pattern string with one of the |
| following sequences: |
| |
| (*BSR_ANYCRLF) CR, LF, or CRLF only |
| (*BSR_UNICODE) any Unicode newline sequence |
| |
| These override the default and the options given to pcre_compile() or |
| pcre_compile2(), but they can be overridden by options given to |
| pcre_exec() or pcre_dfa_exec(). Note that these special settings, which |
| are not Perl-compatible, are recognized only at the very start of a |
| pattern, and that they must be in upper case. If more than one of them |
| is present, the last one is used. They can be combined with a change of |
| newline convention; for example, a pattern can start with: |
| |
| (*ANY)(*BSR_ANYCRLF) |
| |
| They can also be combined with the (*UTF8) or (*UCP) special sequences. |
| Inside a character class, \R is treated as an unrecognized escape |
| sequence, and so matches the letter "R" by default, but causes an error |
| if PCRE_EXTRA is set. |
| |
| Unicode character properties |
| |
| When PCRE is built with Unicode character property support, three addi- |
| tional escape sequences that match characters with specific properties |
| are available. When not in UTF-8 mode, these sequences are of course |
| limited to testing characters whose codepoints are less than 256, but |
| they do work in this mode. The extra escape sequences are: |
| |
| \p{xx} a character with the xx property |
| \P{xx} a character without the xx property |
| \X an extended Unicode sequence |
| |
| The property names represented by xx above are limited to the Unicode |
| script names, the general category properties, "Any", which matches any |
| character (including newline), and some special PCRE properties |
| (described in the next section). Other Perl properties such as "InMu- |
| sicalSymbols" are not currently supported by PCRE. Note that \P{Any} |
| does not match any characters, so always causes a match failure. |
| |
| Sets of Unicode characters are defined as belonging to certain scripts. |
| A character from one of these sets can be matched using a script name. |
| For example: |
| |
| \p{Greek} |
| \P{Han} |
| |
| Those that are not part of an identified script are lumped together as |
| "Common". The current list of scripts is: |
| |
| Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille, |
| Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common, |
| Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyp- |
| tian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek, |
| Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Impe- |
| rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian, |
| Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao, |
| Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam, |
| Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic, |
| Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya, |
| Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian, |
| Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, |
| Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, |
| Ugaritic, Vai, Yi. |
| |
| Each character has exactly one Unicode general category property, spec- |
| ified by a two-letter abbreviation. For compatibility with Perl, nega- |
| tion can be specified by including a circumflex between the opening |
| brace and the property name. For example, \p{^Lu} is the same as |
| \P{Lu}. |
| |
| If only one letter is specified with \p or \P, it includes all the gen- |
| eral category properties that start with that letter. In this case, in |
| the absence of negation, the curly brackets in the escape sequence are |
| optional; these two examples have the same effect: |
| |
| \p{L} |
| \pL |
| |
| The following general category property codes are supported: |
| |
| C Other |
| Cc Control |
| Cf Format |
| Cn Unassigned |
| Co Private use |
| Cs Surrogate |
| |
| L Letter |
| Ll Lower case letter |
| Lm Modifier letter |
| Lo Other letter |
| Lt Title case letter |
| Lu Upper case letter |
| |
| M Mark |
| Mc Spacing mark |
| Me Enclosing mark |
| Mn Non-spacing mark |
| |
| N Number |
| Nd Decimal number |
| Nl Letter number |
| No Other number |
| |
| P Punctuation |
| Pc Connector punctuation |
| Pd Dash punctuation |
| Pe Close punctuation |
| Pf Final punctuation |
| Pi Initial punctuation |
| Po Other punctuation |
| Ps Open punctuation |
| |
| S Symbol |
| Sc Currency symbol |
| Sk Modifier symbol |
| Sm Mathematical symbol |
| So Other symbol |
| |
| Z Separator |
| Zl Line separator |
| Zp Paragraph separator |
| Zs Space separator |
| |
| The special property L& is also supported: it matches a character that |
| has the Lu, Ll, or Lt property, in other words, a letter that is not |
| classified as a modifier or "other". |
| |
| The Cs (Surrogate) property applies only to characters in the range |
| U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see |
| RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check- |
| ing has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in |
| the pcreapi page). Perl does not support the Cs property. |
| |
| The long synonyms for property names that Perl supports (such as |
| \p{Letter}) are not supported by PCRE, nor is it permitted to prefix |
| any of these properties with "Is". |
| |
| No character that is in the Unicode table has the Cn (unassigned) prop- |
| erty. Instead, this property is assumed for any code point that is not |
| in the Unicode table. |
| |
| Specifying caseless matching does not affect these escape sequences. |
| For example, \p{Lu} always matches only upper case letters. |
| |
| The \X escape matches any number of Unicode characters that form an |
| extended Unicode sequence. \X is equivalent to |
| |
| (?>\PM\pM*) |
| |
| That is, it matches a character without the "mark" property, followed |
| by zero or more characters with the "mark" property, and treats the |
| sequence as an atomic group (see below). Characters with the "mark" |
| property are typically accents that affect the preceding character. |
| None of them have codepoints less than 256, so in non-UTF-8 mode \X |
| matches any one character. |
| |
| Note that recent versions of Perl have changed \X to match what Unicode |
| calls an "extended grapheme cluster", which has a more complicated def- |
| inition. |
| |
| Matching characters by Unicode property is not fast, because PCRE has |
| to search a structure that contains data for over fifteen thousand |
| characters. That is why the traditional escape sequences such as \d and |
| \w do not use Unicode properties in PCRE by default, though you can |
| make them do so by setting the PCRE_UCP option for pcre_compile() or by |
| starting the pattern with (*UCP). |
| |
| PCRE's additional properties |
| |
| As well as the standard Unicode properties described in the previous |
| section, PCRE supports four more that make it possible to convert tra- |
| ditional escape sequences such as \w and \s and POSIX character classes |
| to use Unicode properties. PCRE uses these non-standard, non-Perl prop- |
| erties internally when PCRE_UCP is set. They are: |
| |
| Xan Any alphanumeric character |
| Xps Any POSIX space character |
| Xsp Any Perl space character |
| Xwd Any Perl "word" character |
| |
| Xan matches characters that have either the L (letter) or the N (num- |
| ber) property. Xps matches the characters tab, linefeed, vertical tab, |
| formfeed, or carriage return, and any other character that has the Z |
| (separator) property. Xsp is the same as Xps, except that vertical tab |
| is excluded. Xwd matches the same characters as Xan, plus underscore. |
| |
| Resetting the match start |
| |
| The escape sequence \K causes any previously matched characters not to |
| be included in the final matched sequence. For example, the pattern: |
| |
| foo\Kbar |
| |
| matches "foobar", but reports that it has matched "bar". This feature |
| is similar to a lookbehind assertion (described below). However, in |
| this case, the part of the subject before the real match does not have |
| to be of fixed length, as lookbehind assertions do. The use of \K does |
| not interfere with the setting of captured substrings. For example, |
| when the pattern |
| |
| (foo)\Kbar |
| |
| matches "foobar", the first substring is still set to "foo". |
| |
| Perl documents that the use of \K within assertions is "not well |
| defined". In PCRE, \K is acted upon when it occurs inside positive |
| assertions, but is ignored in negative assertions. |
| |
| Simple assertions |
| |
| The final use of backslash is for certain simple assertions. An asser- |
| tion specifies a condition that has to be met at a particular point in |
| a match, without consuming any characters from the subject string. The |
| use of subpatterns for more complicated assertions is described below. |
| The backslashed assertions are: |
| |
| \b matches at a word boundary |
| \B matches when not at a word boundary |
| \A matches at the start of the subject |
| \Z matches at the end of the subject |
| also matches before a newline at the end of the subject |
| \z matches only at the end of the subject |
| \G matches at the first matching position in the subject |
| |
| Inside a character class, \b has a different meaning; it matches the |
| backspace character. If any other of these assertions appears in a |
| character class, by default it matches the corresponding literal char- |
| acter (for example, \B matches the letter B). However, if the |
| PCRE_EXTRA option is set, an "invalid escape sequence" error is gener- |
| ated instead. |
| |
| A word boundary is a position in the subject string where the current |
| character and the previous character do not both match \w or \W (i.e. |
| one matches \w and the other matches \W), or the start or end of the |
| string if the first or last character matches \w, respectively. In |
| UTF-8 mode, the meanings of \w and \W can be changed by setting the |
| PCRE_UCP option. When this is done, it also affects \b and \B. Neither |
| PCRE nor Perl has a separate "start of word" or "end of word" metase- |
| quence. However, whatever follows \b normally determines which it is. |
| For example, the fragment \ba matches "a" at the start of a word. |
| |
| The \A, \Z, and \z assertions differ from the traditional circumflex |
| and dollar (described in the next section) in that they only ever match |
| at the very start and end of the subject string, whatever options are |
| set. Thus, they are independent of multiline mode. These three asser- |
| tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which |
| affect only the behaviour of the circumflex and dollar metacharacters. |
| However, if the startoffset argument of pcre_exec() is non-zero, indi- |
| cating that matching is to start at a point other than the beginning of |
| the subject, \A can never match. The difference between \Z and \z is |
| that \Z matches before a newline at the end of the string as well as at |
| the very end, whereas \z matches only at the end. |
| |
| The \G assertion is true only when the current matching position is at |
| the start point of the match, as specified by the startoffset argument |
| of pcre_exec(). It differs from \A when the value of startoffset is |
| non-zero. By calling pcre_exec() multiple times with appropriate argu- |
| ments, you can mimic Perl's /g option, and it is in this kind of imple- |
| mentation where \G can be useful. |
| |
| Note, however, that PCRE's interpretation of \G, as the start of the |
| current match, is subtly different from Perl's, which defines it as the |
| end of the previous match. In Perl, these can be different when the |
| previously matched string was empty. Because PCRE does just one match |
| at a time, it cannot reproduce this behaviour. |
| |
| If all the alternatives of a pattern begin with \G, the expression is |
| anchored to the starting match position, and the "anchored" flag is set |
| in the compiled regular expression. |
| |
| |
| CIRCUMFLEX AND DOLLAR |
| |
| Outside a character class, in the default matching mode, the circumflex |
| character is an assertion that is true only if the current matching |
| point is at the start of the subject string. If the startoffset argu- |
| ment of pcre_exec() is non-zero, circumflex can never match if the |
| PCRE_MULTILINE option is unset. Inside a character class, circumflex |
| has an entirely different meaning (see below). |
| |
| Circumflex need not be the first character of the pattern if a number |
| of alternatives are involved, but it should be the first thing in each |
| alternative in which it appears if the pattern is ever to match that |
| branch. If all possible alternatives start with a circumflex, that is, |
| if the pattern is constrained to match only at the start of the sub- |
| ject, it is said to be an "anchored" pattern. (There are also other |
| constructs that can cause a pattern to be anchored.) |
| |
| A dollar character is an assertion that is true only if the current |
| matching point is at the end of the subject string, or immediately |
| before a newline at the end of the string (by default). Dollar need not |
| be the last character of the pattern if a number of alternatives are |
| involved, but it should be the last item in any branch in which it |
| appears. Dollar has no special meaning in a character class. |
| |
| The meaning of dollar can be changed so that it matches only at the |
| very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at |
| compile time. This does not affect the \Z assertion. |
| |
| The meanings of the circumflex and dollar characters are changed if the |
| PCRE_MULTILINE option is set. When this is the case, a circumflex |
| matches immediately after internal newlines as well as at the start of |
| the subject string. It does not match after a newline that ends the |
| string. A dollar matches before any newlines in the string, as well as |
| at the very end, when PCRE_MULTILINE is set. When newline is specified |
| as the two-character sequence CRLF, isolated CR and LF characters do |
| not indicate newlines. |
| |
| For example, the pattern /^abc$/ matches the subject string "def\nabc" |
| (where \n represents a newline) in multiline mode, but not otherwise. |
| Consequently, patterns that are anchored in single line mode because |
| all branches start with ^ are not anchored in multiline mode, and a |
| match for circumflex is possible when the startoffset argument of |
| pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if |
| PCRE_MULTILINE is set. |
| |
| Note that the sequences \A, \Z, and \z can be used to match the start |
| and end of the subject in both modes, and if all branches of a pattern |
| start with \A it is always anchored, whether or not PCRE_MULTILINE is |
| set. |
| |
| |
| FULL STOP (PERIOD, DOT) AND \N |
| |
| Outside a character class, a dot in the pattern matches any one charac- |
| ter in the subject string except (by default) a character that signi- |
| fies the end of a line. In UTF-8 mode, the matched character may be |
| more than one byte long. |
| |
| When a line ending is defined as a single character, dot never matches |
| that character; when the two-character sequence CRLF is used, dot does |
| not match CR if it is immediately followed by LF, but otherwise it |
| matches all characters (including isolated CRs and LFs). When any Uni- |
| code line endings are being recognized, dot does not match CR or LF or |
| any of the other line ending characters. |
| |
| The behaviour of dot with regard to newlines can be changed. If the |
| PCRE_DOTALL option is set, a dot matches any one character, without |
| exception. If the two-character sequence CRLF is present in the subject |
| string, it takes two dots to match it. |
| |
| The handling of dot is entirely independent of the handling of circum- |
| flex and dollar, the only relationship being that they both involve |
| newlines. Dot has no special meaning in a character class. |
| |
| The escape sequence \N behaves like a dot, except that it is not |
| affected by the PCRE_DOTALL option. In other words, it matches any |
| character except one that signifies the end of a line. Perl also uses |
| \N to match characters by name; PCRE does not support this. |
| |
| |
| MATCHING A SINGLE BYTE |
| |
| Outside a character class, the escape sequence \C matches any one byte, |
| both in and out of UTF-8 mode. Unlike a dot, it always matches line- |
| ending characters. The feature is provided in Perl in order to match |
| individual bytes in UTF-8 mode, but it is unclear how it can usefully |
| be used. Because \C breaks up characters into individual bytes, match- |
| ing one byte with \C in UTF-8 mode means that the rest of the string |
| may start with a malformed UTF-8 character. This has undefined results, |
| because PCRE assumes that it is dealing with valid UTF-8 strings (and |
| by default it checks this at the start of processing unless the |
| PCRE_NO_UTF8_CHECK option is used). |
| |
| PCRE does not allow \C to appear in lookbehind assertions (described |
| below) in UTF-8 mode, because this would make it impossible to calcu- |
| late the length of the lookbehind. |
| |
| In general, the \C escape sequence is best avoided in UTF-8 mode. How- |
| ever, one way of using it that avoids the problem of malformed UTF-8 |
| characters is to use a lookahead to check the length of the next char- |
| acter, as in this pattern (ignore white space and line breaks): |
| |
| (?| (?=[\x00-\x7f])(\C) | |
| (?=[\x80-\x{7ff}])(\C)(\C) | |
| (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) | |
| (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C)) |
| |
| A group that starts with (?| resets the capturing parentheses numbers |
| in each alternative (see "Duplicate Subpattern Numbers" below). The |
| assertions at the start of each branch check the next UTF-8 character |
| for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The |
| character's individual bytes are then captured by the appropriate num- |
| ber of groups. |
| |
| |
| SQUARE BRACKETS AND CHARACTER CLASSES |
| |
| An opening square bracket introduces a character class, terminated by a |
| closing square bracket. A closing square bracket on its own is not spe- |
| cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set, |
| a lone closing square bracket causes a compile-time error. If a closing |
| square bracket is required as a member of the class, it should be the |
| first data character in the class (after an initial circumflex, if |
| present) or escaped with a backslash. |
| |
| A character class matches a single character in the subject. In UTF-8 |
| mode, the character may be more than one byte long. A matched character |
| must be in the set of characters defined by the class, unless the first |
| character in the class definition is a circumflex, in which case the |
| subject character must not be in the set defined by the class. If a |
| circumflex is actually required as a member of the class, ensure it is |
| not the first character, or escape it with a backslash. |
| |
| For example, the character class [aeiou] matches any lower case vowel, |
| while [^aeiou] matches any character that is not a lower case vowel. |
| Note that a circumflex is just a convenient notation for specifying the |
| characters that are in the class by enumerating those that are not. A |
| class that starts with a circumflex is not an assertion; it still con- |
| sumes a character from the subject string, and therefore it fails if |
| the current pointer is at the end of the string. |
| |
| In UTF-8 mode, characters with values greater than 255 can be included |
| in a class as a literal string of bytes, or by using the \x{ escaping |
| mechanism. |
| |
| When caseless matching is set, any letters in a class represent both |
| their upper case and lower case versions, so for example, a caseless |
| [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not |
| match "A", whereas a caseful version would. In UTF-8 mode, PCRE always |
| understands the concept of case for characters whose values are less |
| than 128, so caseless matching is always possible. For characters with |
| higher values, the concept of case is supported if PCRE is compiled |
| with Unicode property support, but not otherwise. If you want to use |
| caseless matching in UTF8-mode for characters 128 and above, you must |
| ensure that PCRE is compiled with Unicode property support as well as |
| with UTF-8 support. |
| |
| Characters that might indicate line breaks are never treated in any |
| special way when matching character classes, whatever line-ending |
| sequence is in use, and whatever setting of the PCRE_DOTALL and |
| PCRE_MULTILINE options is used. A class such as [^a] always matches one |
| of these characters. |
| |
| The minus (hyphen) character can be used to specify a range of charac- |
| ters in a character class. For example, [d-m] matches any letter |
| between d and m, inclusive. If a minus character is required in a |
| class, it must be escaped with a backslash or appear in a position |
| where it cannot be interpreted as indicating a range, typically as the |
| first or last character in the class. |
| |
| It is not possible to have the literal character "]" as the end charac- |
| ter of a range. A pattern such as [W-]46] is interpreted as a class of |
| two characters ("W" and "-") followed by a literal string "46]", so it |
| would match "W46]" or "-46]". However, if the "]" is escaped with a |
| backslash it is interpreted as the end of range, so [W-\]46] is inter- |
| preted as a class containing a range followed by two other characters. |
| The octal or hexadecimal representation of "]" can also be used to end |
| a range. |
| |
| Ranges operate in the collating sequence of character values. They can |
| also be used for characters specified numerically, for example |
| [\000-\037]. In UTF-8 mode, ranges can include characters whose values |
| are greater than 255, for example [\x{100}-\x{2ff}]. |
| |
| If a range that includes letters is used when caseless matching is set, |
| it matches the letters in either case. For example, [W-c] is equivalent |
| to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if |
| character tables for a French locale are in use, [\xc8-\xcb] matches |
| accented E characters in both cases. In UTF-8 mode, PCRE supports the |
| concept of case for characters with values greater than 128 only when |
| it is compiled with Unicode property support. |
| |
| The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V, |
| \w, and \W may appear in a character class, and add the characters that |
| they match to the class. For example, [\dABCDEF] matches any hexadeci- |
| mal digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of |
| \d, \s, \w and their upper case partners, just as it does when they |
| appear outside a character class, as described in the section entitled |
| "Generic character types" above. The escape sequence \b has a different |
| meaning inside a character class; it matches the backspace character. |
| The sequences \B, \N, \R, and \X are not special inside a character |
| class. Like any other unrecognized escape sequences, they are treated |
| as the literal characters "B", "N", "R", and "X" by default, but cause |
| an error if the PCRE_EXTRA option is set. |
| |
| A circumflex can conveniently be used with the upper case character |
| types to specify a more restricted set of characters than the matching |
| lower case type. For example, the class [^\W_] matches any letter or |
| digit, but not underscore, whereas [\w] includes underscore. A positive |
| character class should be read as "something OR something OR ..." and a |
| negative class as "NOT something AND NOT something AND NOT ...". |
| |
| The only metacharacters that are recognized in character classes are |
| backslash, hyphen (only where it can be interpreted as specifying a |
| range), circumflex (only at the start), opening square bracket (only |
| when it can be interpreted as introducing a POSIX class name - see the |
| next section), and the terminating closing square bracket. However, |
| escaping other non-alphanumeric characters does no harm. |
| |
| |
| POSIX CHARACTER CLASSES |
| |
| Perl supports the POSIX notation for character classes. This uses names |
| enclosed by [: and :] within the enclosing square brackets. PCRE also |
| supports this notation. For example, |
| |
| [01[:alpha:]%] |
| |
| matches "0", "1", any alphabetic character, or "%". The supported class |
| names are: |
| |
| alnum letters and digits |
| alpha letters |
| ascii character codes 0 - 127 |
| blank space or tab only |
| cntrl control characters |
| digit decimal digits (same as \d) |
| graph printing characters, excluding space |
| lower lower case letters |
| print printing characters, including space |
| punct printing characters, excluding letters and digits and space |
| space white space (not quite the same as \s) |
| upper upper case letters |
| word "word" characters (same as \w) |
| xdigit hexadecimal digits |
| |
| The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), |
| and space (32). Notice that this list includes the VT character (code |
| 11). This makes "space" different to \s, which does not include VT (for |
| Perl compatibility). |
| |
| The name "word" is a Perl extension, and "blank" is a GNU extension |
| from Perl 5.8. Another Perl extension is negation, which is indicated |
| by a ^ character after the colon. For example, |
| |
| [12[:^digit:]] |
| |
| matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the |
| POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but |
| these are not supported, and an error is given if they are encountered. |
| |
| By default, in UTF-8 mode, characters with values greater than 128 do |
| not match any of the POSIX character classes. However, if the PCRE_UCP |
| option is passed to pcre_compile(), some of the classes are changed so |
| that Unicode character properties are used. This is achieved by replac- |
| ing the POSIX classes by other sequences, as follows: |
| |
| [:alnum:] becomes \p{Xan} |
| [:alpha:] becomes \p{L} |
| [:blank:] becomes \h |
| [:digit:] becomes \p{Nd} |
| [:lower:] becomes \p{Ll} |
| [:space:] becomes \p{Xps} |
| [:upper:] becomes \p{Lu} |
| [:word:] becomes \p{Xwd} |
| |
| Negated versions, such as [:^alpha:] use \P instead of \p. The other |
| POSIX classes are unchanged, and match only characters with code points |
| less than 128. |
| |
| |
| VERTICAL BAR |
| |
| Vertical bar characters are used to separate alternative patterns. For |
| example, the pattern |
| |
| gilbert|sullivan |
| |
| matches either "gilbert" or "sullivan". Any number of alternatives may |
| appear, and an empty alternative is permitted (matching the empty |
| string). The matching process tries each alternative in turn, from left |
| to right, and the first one that succeeds is used. If the alternatives |
| are within a subpattern (defined below), "succeeds" means matching the |
| rest of the main pattern as well as the alternative in the subpattern. |
| |
| |
| INTERNAL OPTION SETTING |
| |
| The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and |
| PCRE_EXTENDED options (which are Perl-compatible) can be changed from |
| within the pattern by a sequence of Perl option letters enclosed |
| between "(?" and ")". The option letters are |
| |
| i for PCRE_CASELESS |
| m for PCRE_MULTILINE |
| s for PCRE_DOTALL |
| x for PCRE_EXTENDED |
| |
| For example, (?im) sets caseless, multiline matching. It is also possi- |
| ble to unset these options by preceding the letter with a hyphen, and a |
| combined setting and unsetting such as (?im-sx), which sets PCRE_CASE- |
| LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, |
| is also permitted. If a letter appears both before and after the |
| hyphen, the option is unset. |
| |
| The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA |
| can be changed in the same way as the Perl-compatible options by using |
| the characters J, U and X respectively. |
| |
| When one of these option changes occurs at top level (that is, not |
| inside subpattern parentheses), the change applies to the remainder of |
| the pattern that follows. If the change is placed right at the start of |
| a pattern, PCRE extracts it into the global options (and it will there- |
| fore show up in data extracted by the pcre_fullinfo() function). |
| |
| An option change within a subpattern (see below for a description of |
| subpatterns) affects only that part of the subpattern that follows it, |
| so |
| |
| (a(?i)b)c |
| |
| matches abc and aBc and no other strings (assuming PCRE_CASELESS is not |
| used). By this means, options can be made to have different settings |
| in different parts of the pattern. Any changes made in one alternative |
| do carry on into subsequent branches within the same subpattern. For |
| example, |
| |
| (a(?i)b|c) |
| |
| matches "ab", "aB", "c", and "C", even though when matching "C" the |
| first branch is abandoned before the option setting. This is because |
| the effects of option settings happen at compile time. There would be |
| some very weird behaviour otherwise. |
| |
| Note: There are other PCRE-specific options that can be set by the |
| application when the compile or match functions are called. In some |
| cases the pattern can contain special leading sequences such as (*CRLF) |
| to override what the application has set or what has been defaulted. |
| Details are given in the section entitled "Newline sequences" above. |
| There are also the (*UTF8) and (*UCP) leading sequences that can be |
| used to set UTF-8 and Unicode property modes; they are equivalent to |
| setting the PCRE_UTF8 and the PCRE_UCP options, respectively. |
| |
| |
| SUBPATTERNS |
| |
| Subpatterns are delimited by parentheses (round brackets), which can be |
| nested. Turning part of a pattern into a subpattern does two things: |
| |
| 1. It localizes a set of alternatives. For example, the pattern |
| |
| cat(aract|erpillar|) |
| |
| matches "cataract", "caterpillar", or "cat". Without the parentheses, |
| it would match "cataract", "erpillar" or an empty string. |
| |
| 2. It sets up the subpattern as a capturing subpattern. This means |
| that, when the whole pattern matches, that portion of the subject |
| string that matched the subpattern is passed back to the caller via the |
| ovector argument of pcre_exec(). Opening parentheses are counted from |
| left to right (starting from 1) to obtain numbers for the capturing |
| subpatterns. For example, if the string "the red king" is matched |
| against the pattern |
| |
| the ((red|white) (king|queen)) |
| |
| the captured substrings are "red king", "red", and "king", and are num- |
| bered 1, 2, and 3, respectively. |
| |
| The fact that plain parentheses fulfil two functions is not always |
| helpful. There are often times when a grouping subpattern is required |
| without a capturing requirement. If an opening parenthesis is followed |
| by a question mark and a colon, the subpattern does not do any captur- |
| ing, and is not counted when computing the number of any subsequent |
| capturing subpatterns. For example, if the string "the white queen" is |
| matched against the pattern |
| |
| the ((?:red|white) (king|queen)) |
| |
| the captured substrings are "white queen" and "queen", and are numbered |
| 1 and 2. The maximum number of capturing subpatterns is 65535. |
| |
| As a convenient shorthand, if any option settings are required at the |
| start of a non-capturing subpattern, the option letters may appear |
| between the "?" and the ":". Thus the two patterns |
| |
| (?i:saturday|sunday) |
| (?:(?i)saturday|sunday) |
| |
| match exactly the same set of strings. Because alternative branches are |
| tried from left to right, and options are not reset until the end of |
| the subpattern is reached, an option setting in one branch does affect |
| subsequent branches, so the above patterns match "SUNDAY" as well as |
| "Saturday". |
| |
| |
| DUPLICATE SUBPATTERN NUMBERS |
| |
| Perl 5.10 introduced a feature whereby each alternative in a subpattern |
| uses the same numbers for its capturing parentheses. Such a subpattern |
| starts with (?| and is itself a non-capturing subpattern. For example, |
| consider this pattern: |
| |
| (?|(Sat)ur|(Sun))day |
| |
| Because the two alternatives are inside a (?| group, both sets of cap- |
| turing parentheses are numbered one. Thus, when the pattern matches, |
| you can look at captured substring number one, whichever alternative |
| matched. This construct is useful when you want to capture part, but |
| not all, of one of a number of alternatives. Inside a (?| group, paren- |
| theses are numbered as usual, but the number is reset at the start of |
| each branch. The numbers of any capturing parentheses that follow the |
| subpattern start after the highest number used in any branch. The fol- |
| lowing example is taken from the Perl documentation. The numbers under- |
| neath show in which buffer the captured content will be stored. |
| |
| # before ---------------branch-reset----------- after |
| / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x |
| # 1 2 2 3 2 3 4 |
| |
| A back reference to a numbered subpattern uses the most recent value |
| that is set for that number by any subpattern. The following pattern |
| matches "abcabc" or "defdef": |
| |
| /(?|(abc)|(def))\1/ |
| |
| In contrast, a subroutine call to a numbered subpattern always refers |
| to the first one in the pattern with the given number. The following |
| pattern matches "abcabc" or "defabc": |
| |
| /(?|(abc)|(def))(?1)/ |
| |
| If a condition test for a subpattern's having matched refers to a non- |
| unique number, the test is true if any of the subpatterns of that num- |
| ber have matched. |
| |
| An alternative approach to using this "branch reset" feature is to use |
| duplicate named subpatterns, as described in the next section. |
| |
| |
| NAMED SUBPATTERNS |
| |
| Identifying capturing parentheses by number is simple, but it can be |
| very hard to keep track of the numbers in complicated regular expres- |
| sions. Furthermore, if an expression is modified, the numbers may |
| change. To help with this difficulty, PCRE supports the naming of sub- |
| patterns. This feature was not added to Perl until release 5.10. Python |
| had the feature earlier, and PCRE introduced it at release 4.0, using |
| the Python syntax. PCRE now supports both the Perl and the Python syn- |
| tax. Perl allows identically numbered subpatterns to have different |
| names, but PCRE does not. |
| |
| In PCRE, a subpattern can be named in one of three ways: (?<name>...) |
| or (?'name'...) as in Perl, or (?P<name>...) as in Python. References |
| to capturing parentheses from other parts of the pattern, such as back |
| references, recursion, and conditions, can be made by name as well as |
| by number. |
| |
| Names consist of up to 32 alphanumeric characters and underscores. |
| Named capturing parentheses are still allocated numbers as well as |
| names, exactly as if the names were not present. The PCRE API provides |
| function calls for extracting the name-to-number translation table from |
| a compiled pattern. There is also a convenience function for extracting |
| a captured substring by name. |
| |
| By default, a name must be unique within a pattern, but it is possible |
| to relax this constraint by setting the PCRE_DUPNAMES option at compile |
| time. (Duplicate names are also always permitted for subpatterns with |
| the same number, set up as described in the previous section.) Dupli- |
| cate names can be useful for patterns where only one instance of the |
| named parentheses can match. Suppose you want to match the name of a |
| weekday, either as a 3-letter abbreviation or as the full name, and in |
| both cases you want to extract the abbreviation. This pattern (ignoring |
| the line breaks) does the job: |
| |
| (?<DN>Mon|Fri|Sun)(?:day)?| |
| (?<DN>Tue)(?:sday)?| |
| (?<DN>Wed)(?:nesday)?| |
| (?<DN>Thu)(?:rsday)?| |
| (?<DN>Sat)(?:urday)? |
| |
| There are five capturing substrings, but only one is ever set after a |
| match. (An alternative way of solving this problem is to use a "branch |
| reset" subpattern, as described in the previous section.) |
| |
| The convenience function for extracting the data by name returns the |
| substring for the first (and in this example, the only) subpattern of |
| that name that matched. This saves searching to find which numbered |
| subpattern it was. |
| |
| If you make a back reference to a non-unique named subpattern from |
| elsewhere in the pattern, the one that corresponds to the first occur- |
| rence of the name is used. In the absence of duplicate numbers (see the |
| previous section) this is the one with the lowest number. If you use a |
| named reference in a condition test (see the section about conditions |
| below), either to check whether a subpattern has matched, or to check |
| for recursion, all subpatterns with the same name are tested. If the |
| condition is true for any one of them, the overall condition is true. |
| This is the same behaviour as testing by number. For further details of |
| the interfaces for handling named subpatterns, see the pcreapi documen- |
| tation. |
| |
| Warning: You cannot use different names to distinguish between two sub- |
| patterns with the same number because PCRE uses only the numbers when |
| matching. For this reason, an error is given at compile time if differ- |
| ent names are given to subpatterns with the same number. However, you |
| can give the same name to subpatterns with the same number, even when |
| PCRE_DUPNAMES is not set. |
| |
| |
| REPETITION |
| |
| Repetition is specified by quantifiers, which can follow any of the |
| following items: |
| |
| a literal data character |
| the dot metacharacter |
| the \C escape sequence |
| the \X escape sequence (in UTF-8 mode with Unicode properties) |
| the \R escape sequence |
| an escape such as \d or \pL that matches a single character |
| a character class |
| a back reference (see next section) |
| a parenthesized subpattern (including assertions) |
| a subroutine call to a subpattern (recursive or otherwise) |
| |
| The general repetition quantifier specifies a minimum and maximum num- |
| ber of permitted matches, by giving the two numbers in curly brackets |
| (braces), separated by a comma. The numbers must be less than 65536, |
| and the first must be less than or equal to the second. For example: |
| |
| z{2,4} |
| |
| matches "zz", "zzz", or "zzzz". A closing brace on its own is not a |
| special character. If the second number is omitted, but the comma is |
| present, there is no upper limit; if the second number and the comma |
| are both omitted, the quantifier specifies an exact number of required |
| matches. Thus |
| |
| [aeiou]{3,} |
| |
| matches at least 3 successive vowels, but may match many more, while |
| |
| \d{8} |
| |
| matches exactly 8 digits. An opening curly bracket that appears in a |
| position where a quantifier is not allowed, or one that does not match |
| the syntax of a quantifier, is taken as a literal character. For exam- |
| ple, {,6} is not a quantifier, but a literal string of four characters. |
| |
| In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to |
| individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char- |
| acters, each of which is represented by a two-byte sequence. Similarly, |
| when Unicode property support is available, \X{3} matches three Unicode |
| extended sequences, each of which may be several bytes long (and they |
| may be of different lengths). |
| |
| The quantifier {0} is permitted, causing the expression to behave as if |
| the previous item and the quantifier were not present. This may be use- |
| ful for subpatterns that are referenced as subroutines from elsewhere |
| in the pattern (but see also the section entitled "Defining subpatterns |
| for use by reference only" below). Items other than subpatterns that |
| have a {0} quantifier are omitted from the compiled pattern. |
| |
| For convenience, the three most common quantifiers have single-charac- |
| ter abbreviations: |
| |
| * is equivalent to {0,} |
| + is equivalent to {1,} |
| ? is equivalent to {0,1} |
| |
| It is possible to construct infinite loops by following a subpattern |
| that can match no characters with a quantifier that has no upper limit, |
| for example: |
| |
| (a?)* |
| |
| Earlier versions of Perl and PCRE used to give an error at compile time |
| for such patterns. However, because there are cases where this can be |
| useful, such patterns are now accepted, but if any repetition of the |
| subpattern does in fact match no characters, the loop is forcibly bro- |
| ken. |
| |
| By default, the quantifiers are "greedy", that is, they match as much |
| as possible (up to the maximum number of permitted times), without |
| causing the rest of the pattern to fail. The classic example of where |
| this gives problems is in trying to match comments in C programs. These |
| appear between /* and */ and within the comment, individual * and / |
| characters may appear. An attempt to match C comments by applying the |
| pattern |
| |
| /\*.*\*/ |
| |
| to the string |
| |
| /* first comment */ not comment /* second comment */ |
| |
| fails, because it matches the entire string owing to the greediness of |
| the .* item. |
| |
| However, if a quantifier is followed by a question mark, it ceases to |
| be greedy, and instead matches the minimum number of times possible, so |
| the pattern |
| |
| /\*.*?\*/ |
| |
| does the right thing with the C comments. The meaning of the various |
| quantifiers is not otherwise changed, just the preferred number of |
| matches. Do not confuse this use of question mark with its use as a |
| quantifier in its own right. Because it has two uses, it can sometimes |
| appear doubled, as in |
| |
| \d??\d |
| |
| which matches one digit by preference, but can match two if that is the |
| only way the rest of the pattern matches. |
| |
| If the PCRE_UNGREEDY option is set (an option that is not available in |
| Perl), the quantifiers are not greedy by default, but individual ones |
| can be made greedy by following them with a question mark. In other |
| words, it inverts the default behaviour. |
| |
| When a parenthesized subpattern is quantified with a minimum repeat |
| count that is greater than 1 or with a limited maximum, more memory is |
| required for the compiled pattern, in proportion to the size of the |
| minimum or maximum. |
| |
| If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv- |
| alent to Perl's /s) is set, thus allowing the dot to match newlines, |
| the pattern is implicitly anchored, because whatever follows will be |
| tried against every character position in the subject string, so there |
| is no point in retrying the overall match at any position after the |
| first. PCRE normally treats such a pattern as though it were preceded |
| by \A. |
| |
| In cases where it is known that the subject string contains no new- |
| lines, it is worth setting PCRE_DOTALL in order to obtain this opti- |
| mization, or alternatively using ^ to indicate anchoring explicitly. |
| |
| However, there is one situation where the optimization cannot be used. |
| When .* is inside capturing parentheses that are the subject of a back |
| reference elsewhere in the pattern, a match at the start may fail where |
| a later one succeeds. Consider, for example: |
| |
| (.*)abc\1 |
| |
| If the subject is "xyz123abc123" the match point is the fourth charac- |
| ter. For this reason, such a pattern is not implicitly anchored. |
| |
| When a capturing subpattern is repeated, the value captured is the sub- |
| string that matched the final iteration. For example, after |
| |
| (tweedle[dume]{3}\s*)+ |
| |
| has matched "tweedledum tweedledee" the value of the captured substring |
| is "tweedledee". However, if there are nested capturing subpatterns, |
| the corresponding captured values may have been set in previous itera- |
| tions. For example, after |
| |
| /(a|(b))+/ |
| |
| matches "aba" the value of the second captured substring is "b". |
| |
| |
| ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS |
| |
| With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") |
| repetition, failure of what follows normally causes the repeated item |
| to be re-evaluated to see if a different number of repeats allows the |
| rest of the pattern to match. Sometimes it is useful to prevent this, |
| either to change the nature of the match, or to cause it fail earlier |
| than it otherwise might, when the author of the pattern knows there is |
| no point in carrying on. |
| |
| Consider, for example, the pattern \d+foo when applied to the subject |
| line |
| |
| 123456bar |
| |
| After matching all 6 digits and then failing to match "foo", the normal |
| action of the matcher is to try again with only 5 digits matching the |
| \d+ item, and then with 4, and so on, before ultimately failing. |
| "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides |
| the means for specifying that once a subpattern has matched, it is not |
| to be re-evaluated in this way. |
| |
| If we use atomic grouping for the previous example, the matcher gives |
| up immediately on failing to match "foo" the first time. The notation |
| is a kind of special parenthesis, starting with (?> as in this example: |
| |
| (?>\d+)foo |
| |
| This kind of parenthesis "locks up" the part of the pattern it con- |
| tains once it has matched, and a failure further into the pattern is |
| prevented from backtracking into it. Backtracking past it to previous |
| items, however, works as normal. |
| |
| An alternative description is that a subpattern of this type matches |
| the string of characters that an identical standalone pattern would |
| match, if anchored at the current point in the subject string. |
| |
| Atomic grouping subpatterns are not capturing subpatterns. Simple cases |
| such as the above example can be thought of as a maximizing repeat that |
| must swallow everything it can. So, while both \d+ and \d+? are pre- |
| pared to adjust the number of digits they match in order to make the |
| rest of the pattern match, (?>\d+) can only match an entire sequence of |
| digits. |
| |
| Atomic groups in general can of course contain arbitrarily complicated |
| subpatterns, and can be nested. However, when the subpattern for an |
| atomic group is just a single repeated item, as in the example above, a |
| simpler notation, called a "possessive quantifier" can be used. This |
| consists of an additional + character following a quantifier. Using |
| this notation, the previous example can be rewritten as |
| |
| \d++foo |
| |
| Note that a possessive quantifier can be used with an entire group, for |
| example: |
| |
| (abc|xyz){2,3}+ |
| |
| Possessive quantifiers are always greedy; the setting of the |
| PCRE_UNGREEDY option is ignored. They are a convenient notation for the |
| simpler forms of atomic group. However, there is no difference in the |
| meaning of a possessive quantifier and the equivalent atomic group, |
| though there may be a performance difference; possessive quantifiers |
| should be slightly faster. |
| |
| The possessive quantifier syntax is an extension to the Perl 5.8 syn- |
| tax. Jeffrey Friedl originated the idea (and the name) in the first |
| edition of his book. Mike McCloskey liked it, so implemented it when he |
| built Sun's Java package, and PCRE copied it from there. It ultimately |
| found its way into Perl at release 5.10. |
| |
| PCRE has an optimization that automatically "possessifies" certain sim- |
| ple pattern constructs. For example, the sequence A+B is treated as |
| A++B because there is no point in backtracking into a sequence of A's |
| when B must follow. |
| |
| When a pattern contains an unlimited repeat inside a subpattern that |
| can itself be repeated an unlimited number of times, the use of an |
| atomic group is the only way to avoid some failing matches taking a |
| very long time indeed. The pattern |
| |
| (\D+|<\d+>)*[!?] |
| |
| matches an unlimited number of substrings that either consist of non- |
| digits, or digits enclosed in <>, followed by either ! or ?. When it |
| matches, it runs quickly. However, if it is applied to |
| |
| aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa |
| |
| it takes a long time before reporting failure. This is because the |
| string can be divided between the internal \D+ repeat and the external |
| * repeat in a large number of ways, and all have to be tried. (The |
| example uses [!?] rather than a single character at the end, because |
| both PCRE and Perl have an optimization that allows for fast failure |
| when a single character is used. They remember the last single charac- |
| ter that is required for a match, and fail early if it is not present |
| in the string.) If the pattern is changed so that it uses an atomic |
| group, like this: |
| |
| ((?>\D+)|<\d+>)*[!?] |
| |
| sequences of non-digits cannot be broken, and failure happens quickly. |
| |
| |
| BACK REFERENCES |
| |
| Outside a character class, a backslash followed by a digit greater than |
| 0 (and possibly further digits) is a back reference to a capturing sub- |
| pattern earlier (that is, to its left) in the pattern, provided there |
| have been that many previous capturing left parentheses. |
| |
| However, if the decimal number following the backslash is less than 10, |
| it is always taken as a back reference, and causes an error only if |
| there are not that many capturing left parentheses in the entire pat- |
| tern. In other words, the parentheses that are referenced need not be |
| to the left of the reference for numbers less than 10. A "forward back |
| reference" of this type can make sense when a repetition is involved |
| and the subpattern to the right has participated in an earlier itera- |
| tion. |
| |
| It is not possible to have a numerical "forward back reference" to a |
| subpattern whose number is 10 or more using this syntax because a |
| sequence such as \50 is interpreted as a character defined in octal. |
| See the subsection entitled "Non-printing characters" above for further |
| details of the handling of digits following a backslash. There is no |
| such problem when named parentheses are used. A back reference to any |
| subpattern is possible using named parentheses (see below). |
| |
| Another way of avoiding the ambiguity inherent in the use of digits |
| following a backslash is to use the \g escape sequence. This escape |
| must be followed by an unsigned number or a negative number, optionally |
| enclosed in braces. These examples are all identical: |
| |
| (ring), \1 |
| (ring), \g1 |
| (ring), \g{1} |
| |
| An unsigned number specifies an absolute reference without the ambigu- |
| ity that is present in the older syntax. It is also useful when literal |
| digits follow the reference. A negative number is a relative reference. |
| Consider this example: |
| |
| (abc(def)ghi)\g{-1} |
| |
| The sequence \g{-1} is a reference to the most recently started captur- |
| ing subpattern before \g, that is, is it equivalent to \2 in this exam- |
| ple. Similarly, \g{-2} would be equivalent to \1. The use of relative |
| references can be helpful in long patterns, and also in patterns that |
| are created by joining together fragments that contain references |
| within themselves. |
| |
| A back reference matches whatever actually matched the capturing sub- |
| pattern in the current subject string, rather than anything matching |
| the subpattern itself (see "Subpatterns as subroutines" below for a way |
| of doing that). So the pattern |
| |
| (sens|respons)e and \1ibility |
| |
| matches "sense and sensibility" and "response and responsibility", but |
| not "sense and responsibility". If caseful matching is in force at the |
| time of the back reference, the case of letters is relevant. For exam- |
| ple, |
| |
| ((?i)rah)\s+\1 |
| |
| matches "rah rah" and "RAH RAH", but not "RAH rah", even though the |
| original capturing subpattern is matched caselessly. |
| |
| There are several different ways of writing back references to named |
| subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or |
| \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's |
| unified back reference syntax, in which \g can be used for both numeric |
| and named references, is also supported. We could rewrite the above |
| example in any of the following ways: |
| |
| (?<p1>(?i)rah)\s+\k<p1> |
| (?'p1'(?i)rah)\s+\k{p1} |
| (?P<p1>(?i)rah)\s+(?P=p1) |
| (?<p1>(?i)rah)\s+\g{p1} |
| |
| A subpattern that is referenced by name may appear in the pattern |
| before or after the reference. |
| |
| There may be more than one back reference to the same subpattern. If a |
| subpattern has not actually been used in a particular match, any back |
| references to it always fail by default. For example, the pattern |
| |
| (a|(bc))\2 |
| |
| always fails if it starts to match "a" rather than "bc". However, if |
| the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer- |
| ence to an unset value matches an empty string. |
| |
| Because there may be many capturing parentheses in a pattern, all dig- |
| its following a backslash are taken as part of a potential back refer- |
| ence number. If the pattern continues with a digit character, some |
| delimiter must be used to terminate the back reference. If the |
| PCRE_EXTENDED option is set, this can be whitespace. Otherwise, the \g{ |
| syntax or an empty comment (see "Comments" below) can be used. |
| |
| Recursive back references |
| |
| A back reference that occurs inside the parentheses to which it refers |
| fails when the subpattern is first used, so, for example, (a\1) never |
| matches. However, such references can be useful inside repeated sub- |
| patterns. For example, the pattern |
| |
| (a|b\1)+ |
| |
| matches any number of "a"s and also "aba", "ababbaa" etc. At each iter- |
| ation of the subpattern, the back reference matches the character |
| string corresponding to the previous iteration. In order for this to |
| work, the pattern must be such that the first iteration does not need |
| to match the back reference. This can be done using alternation, as in |
| the example above, or by a quantifier with a minimum of zero. |
| |
| Back references of this type cause the group that they reference to be |
| treated as an atomic group. Once the whole group has been matched, a |
| subsequent matching failure cannot cause backtracking into the middle |
| of the group. |
| |
| |
| ASSERTIONS |
| |
| An assertion is a test on the characters following or preceding the |
| current matching point that does not actually consume any characters. |
| The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are |
| described above. |
| |
| More complicated assertions are coded as subpatterns. There are two |
| kinds: those that look ahead of the current position in the subject |
| string, and those that look behind it. An assertion subpattern is |
| matched in the normal way, except that it does not cause the current |
| matching position to be changed. |
| |
| Assertion subpatterns are not capturing subpatterns. If such an asser- |
| tion contains capturing subpatterns within it, these are counted for |
| the purposes of numbering the capturing subpatterns in the whole pat- |
| tern. However, substring capturing is carried out only for positive |
| assertions, because it does not make sense for negative assertions. |
| |
| For compatibility with Perl, assertion subpatterns may be repeated; |
| though it makes no sense to assert the same thing several times, the |
| side effect of capturing parentheses may occasionally be useful. In |
| practice, there only three cases: |
| |
| (1) If the quantifier is {0}, the assertion is never obeyed during |
| matching. However, it may contain internal capturing parenthesized |
| groups that are called from elsewhere via the subroutine mechanism. |
| |
| (2) If quantifier is {0,n} where n is greater than zero, it is treated |
| as if it were {0,1}. At run time, the rest of the pattern match is |
| tried with and without the assertion, the order depending on the greed- |
| iness of the quantifier. |
| |
| (3) If the minimum repetition is greater than zero, the quantifier is |
| ignored. The assertion is obeyed just once when encountered during |
| matching. |
| |
| Lookahead assertions |
| |
| Lookahead assertions start with (?= for positive assertions and (?! for |
| negative assertions. For example, |
| |
| \w+(?=;) |
| |
| matches a word followed by a semicolon, but does not include the semi- |
| colon in the match, and |
| |
| foo(?!bar) |
| |
| matches any occurrence of "foo" that is not followed by "bar". Note |
| that the apparently similar pattern |
| |
| (?!foo)bar |
| |
| does not find an occurrence of "bar" that is preceded by something |
| other than "foo"; it finds any occurrence of "bar" whatsoever, because |
| the assertion (?!foo) is always true when the next three characters are |
| "bar". A lookbehind assertion is needed to achieve the other effect. |
| |
| If you want to force a matching failure at some point in a pattern, the |
| most convenient way to do it is with (?!) because an empty string |
| always matches, so an assertion that requires there not to be an empty |
| string must always fail. The backtracking control verb (*FAIL) or (*F) |
| is a synonym for (?!). |
| |
| Lookbehind assertions |
| |
| Lookbehind assertions start with (?<= for positive assertions and (?<! |
| for negative assertions. For example, |
| |
| (?<!foo)bar |
| |
| does find an occurrence of "bar" that is not preceded by "foo". The |
| contents of a lookbehind assertion are restricted such that all the |
| strings it matches must have a fixed length. However, if there are sev- |
| eral top-level alternatives, they do not all have to have the same |
| fixed length. Thus |
| |
| (?<=bullock|donkey) |
| |
| is permitted, but |
| |
| (?<!dogs?|cats?) |
| |
| causes an error at compile time. Branches that match different length |
| strings are permitted only at the top level of a lookbehind assertion. |
| This is an extension compared with Perl, which requires all branches to |
| match the same length of string. An assertion such as |
| |
| (?<=ab(c|de)) |
| |
| is not permitted, because its single top-level branch can match two |
| different lengths, but it is acceptable to PCRE if rewritten to use two |
| top-level branches: |
| |
| (?<=abc|abde) |
| |
| In some cases, the escape sequence \K (see above) can be used instead |
| of a lookbehind assertion to get round the fixed-length restriction. |
| |
| The implementation of lookbehind assertions is, for each alternative, |
| to temporarily move the current position back by the fixed length and |
| then try to match. If there are insufficient characters before the cur- |
| rent position, the assertion fails. |
| |
| In UTF-8 mode, PCRE does not allow the \C escape (which matches a sin- |
| gle byte, even in UTF-8 mode) to appear in lookbehind assertions, |
| because it makes it impossible to calculate the length of the lookbe- |
| hind. The \X and \R escapes, which can match different numbers of |
| bytes, are also not permitted. |
| |
| "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in |
| lookbehinds, as long as the subpattern matches a fixed-length string. |
| Recursion, however, is not supported. |
| |
| Possessive quantifiers can be used in conjunction with lookbehind |
| assertions to specify efficient matching of fixed-length strings at the |
| end of subject strings. Consider a simple pattern such as |
| |
| abcd$ |
| |
| when applied to a long string that does not match. Because matching |
| proceeds from left to right, PCRE will look for each "a" in the subject |
| and then see if what follows matches the rest of the pattern. If the |
| pattern is specified as |
| |
| ^.*abcd$ |
| |
| the initial .* matches the entire string at first, but when this fails |
| (because there is no following "a"), it backtracks to match all but the |
| last character, then all but the last two characters, and so on. Once |
| again the search for "a" covers the entire string, from right to left, |
| so we are no better off. However, if the pattern is written as |
| |
| ^.*+(?<=abcd) |
| |
| there can be no backtracking for the .*+ item; it can match only the |
| entire string. The subsequent lookbehind assertion does a single test |
| on the last four characters. If it fails, the match fails immediately. |
| For long strings, this approach makes a significant difference to the |
| processing time. |
| |
| Using multiple assertions |
| |
| Several assertions (of any sort) may occur in succession. For example, |
| |
| (?<=\d{3})(?<!999)foo |
| |
| matches "foo" preceded by three digits that are not "999". Notice that |
| each of the assertions is applied independently at the same point in |
| the subject string. First there is a check that the previous three |
| characters are all digits, and then there is a check that the same |
| three characters are not "999". This pattern does not match "foo" pre- |
| ceded by six characters, the first of which are digits and the last |
| three of which are not "999". For example, it doesn't match "123abc- |
| foo". A pattern to do that is |
| |
| (?<=\d{3}...)(?<!999)foo |
| |
| This time the first assertion looks at the preceding six characters, |
| checking that the first three are digits, and then the second assertion |
| checks that the preceding three characters are not "999". |
| |
| Assertions can be nested in any combination. For example, |
| |
| (?<=(?<!foo)bar)baz |
| |
| matches an occurrence of "baz" that is preceded by "bar" which in turn |
| is not preceded by "foo", while |
| |
| (?<=\d{3}(?!999)...)foo |
| |
| is another pattern that matches "foo" preceded by three digits and any |
| three characters that are not "999". |
| |
| |
| CONDITIONAL SUBPATTERNS |
| |
| It is possible to cause the matching process to obey a subpattern con- |
| ditionally or to choose between two alternative subpatterns, depending |
| on the result of an assertion, or whether a specific capturing subpat- |
| tern has already been matched. The two possible forms of conditional |
| subpattern are: |
| |
| (?(condition)yes-pattern) |
| (?(condition)yes-pattern|no-pattern) |
| |
| If the condition is satisfied, the yes-pattern is used; otherwise the |
| no-pattern (if present) is used. If there are more than two alterna- |
| tives in the subpattern, a compile-time error occurs. Each of the two |
| alternatives may itself contain nested subpatterns of any form, includ- |
| ing conditional subpatterns; the restriction to two alternatives |
| applies only at the level of the condition. This pattern fragment is an |
| example where the alternatives are complex: |
| |
| (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) |
| |
| |
| There are four kinds of condition: references to subpatterns, refer- |
| ences to recursion, a pseudo-condition called DEFINE, and assertions. |
| |
| Checking for a used subpattern by number |
| |
| If the text between the parentheses consists of a sequence of digits, |
| the condition is true if a capturing subpattern of that number has pre- |
| viously matched. If there is more than one capturing subpattern with |
| the same number (see the earlier section about duplicate subpattern |
| numbers), the condition is true if any of them have matched. An alter- |
| native notation is to precede the digits with a plus or minus sign. In |
| this case, the subpattern number is relative rather than absolute. The |
| most recently opened parentheses can be referenced by (?(-1), the next |
| most recent by (?(-2), and so on. Inside loops it can also make sense |
| to refer to subsequent groups. The next parentheses to be opened can be |
| referenced as (?(+1), and so on. (The value zero in any of these forms |
| is not used; it provokes a compile-time error.) |
| |
| Consider the following pattern, which contains non-significant white |
| space to make it more readable (assume the PCRE_EXTENDED option) and to |
| divide it into three parts for ease of discussion: |
| |
| ( \( )? [^()]+ (?(1) \) ) |
| |
| The first part matches an optional opening parenthesis, and if that |
| character is present, sets it as the first captured substring. The sec- |
| ond part matches one or more characters that are not parentheses. The |
| third part is a conditional subpattern that tests whether or not the |
| first set of parentheses matched. If they did, that is, if subject |
| started with an opening parenthesis, the condition is true, and so the |
| yes-pattern is executed and a closing parenthesis is required. Other- |
| wise, since no-pattern is not present, the subpattern matches nothing. |
| In other words, this pattern matches a sequence of non-parentheses, |
| optionally enclosed in parentheses. |
| |
| If you were embedding this pattern in a larger one, you could use a |
| relative reference: |
| |
| ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... |
| |
| This makes the fragment independent of the parentheses in the larger |
| pattern. |
| |
| Checking for a used subpattern by name |
| |
| Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a |
| used subpattern by name. For compatibility with earlier versions of |
| PCRE, which had this facility before Perl, the syntax (?(name)...) is |
| also recognized. However, there is a possible ambiguity with this syn- |
| tax, because subpattern names may consist entirely of digits. PCRE |
| looks first for a named subpattern; if it cannot find one and the name |
| consists entirely of digits, PCRE looks for a subpattern of that num- |
| ber, which must be greater than zero. Using subpattern names that con- |
| sist entirely of digits is not recommended. |
| |
| Rewriting the above example to use a named subpattern gives this: |
| |
| (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) |
| |
| If the name used in a condition of this kind is a duplicate, the test |
| is applied to all subpatterns of the same name, and is true if any one |
| of them has matched. |
| |
| Checking for pattern recursion |
| |
| If the condition is the string (R), and there is no subpattern with the |
| name R, the condition is true if a recursive call to the whole pattern |
| or any subpattern has been made. If digits or a name preceded by amper- |
| sand follow the letter R, for example: |
| |
| (?(R3)...) or (?(R&name)...) |
| |
| the condition is true if the most recent recursion is into a subpattern |
| whose number or name is given. This condition does not check the entire |
| recursion stack. If the name used in a condition of this kind is a |
| duplicate, the test is applied to all subpatterns of the same name, and |
| is true if any one of them is the most recent recursion. |
| |
| At "top level", all these recursion test conditions are false. The |
| syntax for recursive patterns is described below. |
| |
| Defining subpatterns for use by reference only |
| |
| If the condition is the string (DEFINE), and there is no subpattern |
| with the name DEFINE, the condition is always false. In this case, |
| there may be only one alternative in the subpattern. It is always |
| skipped if control reaches this point in the pattern; the idea of |
| DEFINE is that it can be used to define subroutines that can be refer- |
| enced from elsewhere. (The use of subroutines is described below.) For |
| example, a pattern to match an IPv4 address such as "192.168.23.245" |
| could be written like this (ignore whitespace and line breaks): |
| |
| (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) |
| \b (?&byte) (\.(?&byte)){3} \b |
| |
| The first part of the pattern is a DEFINE group inside which a another |
| group named "byte" is defined. This matches an individual component of |
| an IPv4 address (a number less than 256). When matching takes place, |
| this part of the pattern is skipped because DEFINE acts like a false |
| condition. The rest of the pattern uses references to the named group |
| to match the four dot-separated components of an IPv4 address, insist- |
| ing on a word boundary at each end. |
| |
| Assertion conditions |
| |
| If the condition is not in any of the above formats, it must be an |
| assertion. This may be a positive or negative lookahead or lookbehind |
| assertion. Consider this pattern, again containing non-significant |
| white space, and with the two alternatives on the second line: |
| |
| (?(?=[^a-z]*[a-z]) |
| \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) |
| |
| The condition is a positive lookahead assertion that matches an |
| optional sequence of non-letters followed by a letter. In other words, |
| it tests for the presence of at least one letter in the subject. If a |
| letter is found, the subject is matched against the first alternative; |
| otherwise it is matched against the second. This pattern matches |
| strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are |
| letters and dd are digits. |
| |
| |
| COMMENTS |
| |
| There are two ways of including comments in patterns that are processed |
| by PCRE. In both cases, the start of the comment must not be in a char- |
| acter class, nor in the middle of any other sequence of related charac- |
| ters such as (?: or a subpattern name or number. The characters that |
| make up a comment play no part in the pattern matching. |
| |
| The sequence (?# marks the start of a comment that continues up to the |
| next closing parenthesis. Nested parentheses are not permitted. If the |
| PCRE_EXTENDED option is set, an unescaped # character also introduces a |
| comment, which in this case continues to immediately after the next |
| newline character or character sequence in the pattern. Which charac- |
| ters are interpreted as newlines is controlled by the options passed to |
| pcre_compile() or by a special sequence at the start of the pattern, as |
| described in the section entitled "Newline conventions" above. Note |
| that the end of this type of comment is a literal newline sequence in |
| the pattern; escape sequences that happen to represent a newline do not |
| count. For example, consider this pattern when PCRE_EXTENDED is set, |
| and the default newline convention is in force: |
| |
| abc #comment \n still comment |
| |
| On encountering the # character, pcre_compile() skips along, looking |
| for a newline in the pattern. The sequence \n is still literal at this |
| stage, so it does not terminate the comment. Only an actual character |
| with the code value 0x0a (the default newline) does so. |
| |
| |
| RECURSIVE PATTERNS |
| |
| Consider the problem of matching a string in parentheses, allowing for |
| unlimited nested parentheses. Without the use of recursion, the best |
| that can be done is to use a pattern that matches up to some fixed |
| depth of nesting. It is not possible to handle an arbitrary nesting |
| depth. |
| |
| For some time, Perl has provided a facility that allows regular expres- |
| sions to recurse (amongst other things). It does this by interpolating |
| Perl code in the expression at run time, and the code can refer to the |
| expression itself. A Perl pattern using code interpolation to solve the |
| parentheses problem can be created like this: |
| |
| $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; |
| |
| The (?p{...}) item interpolates Perl code at run time, and in this case |
| refers recursively to the pattern in which it appears. |
| |
| Obviously, PCRE cannot support the interpolation of Perl code. Instead, |
| it supports special syntax for recursion of the entire pattern, and |
| also for individual subpattern recursion. After its introduction in |
| PCRE and Python, this kind of recursion was subsequently introduced |
| into Perl at release 5.10. |
| |
| A special item that consists of (? followed by a number greater than |
| zero and a closing parenthesis is a recursive subroutine call of the |
| subpattern of the given number, provided that it occurs inside that |
| subpattern. (If not, it is a non-recursive subroutine call, which is |
| described in the next section.) The special item (?R) or (?0) is a |
| recursive call of the entire regular expression. |
| |
| This PCRE pattern solves the nested parentheses problem (assume the |
| PCRE_EXTENDED option is set so that white space is ignored): |
| |
| \( ( [^()]++ | (?R) )* \) |
| |
| First it matches an opening parenthesis. Then it matches any number of |
| substrings which can either be a sequence of non-parentheses, or a |
| recursive match of the pattern itself (that is, a correctly parenthe- |
| sized substring). Finally there is a closing parenthesis. Note the use |
| of a possessive quantifier to avoid backtracking into sequences of non- |
| parentheses. |
| |
| If this were part of a larger pattern, you would not want to recurse |
| the entire pattern, so instead you could use this: |
| |
| ( \( ( [^()]++ | (?1) )* \) ) |
| |
| We have put the pattern into parentheses, and caused the recursion to |
| refer to them instead of the whole pattern. |
| |
| In a larger pattern, keeping track of parenthesis numbers can be |
| tricky. This is made easier by the use of relative references. Instead |
| of (?1) in the pattern above you can write (?-2) to refer to the second |
| most recently opened parentheses preceding the recursion. In other |
| words, a negative number counts capturing parentheses leftwards from |
| the point at which it is encountered. |
| |
| It is also possible to refer to subsequently opened parentheses, by |
| writing references such as (?+2). However, these cannot be recursive |
| because the reference is not inside the parentheses that are refer- |
| enced. They are always non-recursive subroutine calls, as described in |
| the next section. |
| |
| An alternative approach is to use named parentheses instead. The Perl |
| syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also |
| supported. We could rewrite the above example as follows: |
| |
| (?<pn> \( ( [^()]++ | (?&pn) )* \) ) |
| |
| If there is more than one subpattern with the same name, the earliest |
| one is used. |
| |
| This particular example pattern that we have been looking at contains |
| nested unlimited repeats, and so the use of a possessive quantifier for |
| matching strings of non-parentheses is important when applying the pat- |
| tern to strings that do not match. For example, when this pattern is |
| applied to |
| |
| (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() |
| |
| it yields "no match" quickly. However, if a possessive quantifier is |
| not used, the match runs for a very long time indeed because there are |
| so many different ways the + and * repeats can carve up the subject, |
| and all have to be tested before failure can be reported. |
| |
| At the end of a match, the values of capturing parentheses are those |
| from the outermost level. If you want to obtain intermediate values, a |
| callout function can be used (see below and the pcrecallout documenta- |
| tion). If the pattern above is matched against |
| |
| (ab(cd)ef) |
| |
| the value for the inner capturing parentheses (numbered 2) is "ef", |
| which is the last value taken on at the top level. If a capturing sub- |
| pattern is not matched at the top level, its final captured value is |
| unset, even if it was (temporarily) set at a deeper level during the |
| matching process. |
| |
| If there are more than 15 capturing parentheses in a pattern, PCRE has |
| to obtain extra memory to store data during a recursion, which it does |
| by using pcre_malloc, freeing it via pcre_free afterwards. If no memory |
| can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. |
| |
| Do not confuse the (?R) item with the condition (R), which tests for |
| recursion. Consider this pattern, which matches text in angle brack- |
| ets, allowing for arbitrary nesting. Only digits are allowed in nested |
| brackets (that is, when recursing), whereas any characters are permit- |
| ted at the outer level. |
| |
| < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > |
| |
| In this pattern, (?(R) is the start of a conditional subpattern, with |
| two different alternatives for the recursive and non-recursive cases. |
| The (?R) item is the actual recursive call. |
| |
| Differences in recursion processing between PCRE and Perl |
| |
| Recursion processing in PCRE differs from Perl in two important ways. |
| In PCRE (like Python, but unlike Perl), a recursive subpattern call is |
| always treated as an atomic group. That is, once it has matched some of |
| the subject string, it is never re-entered, even if it contains untried |
| alternatives and there is a subsequent matching failure. This can be |
| illustrated by the following pattern, which purports to match a palin- |
| dromic string that contains an odd number of characters (for example, |
| "a", "aba", "abcba", "abcdcba"): |
| |
| ^(.|(.)(?1)\2)$ |
| |
| The idea is that it either matches a single character, or two identical |
| characters surrounding a sub-palindrome. In Perl, this pattern works; |
| in PCRE it does not if the pattern is longer than three characters. |
| Consider the subject string "abcba": |
| |
| At the top level, the first character is matched, but as it is not at |
| the end of the string, the first alternative fails; the second alterna- |
| tive is taken and the recursion kicks in. The recursive call to subpat- |
| tern 1 successfully matches the next character ("b"). (Note that the |
| beginning and end of line tests are not part of the recursion). |
| |
| Back at the top level, the next character ("c") is compared with what |
| subpattern 2 matched, which was "a". This fails. Because the recursion |
| is treated as an atomic group, there are now no backtracking points, |
| and so the entire match fails. (Perl is able, at this point, to re- |
| enter the recursion and try the second alternative.) However, if the |
| pattern is written with the alternatives in the other order, things are |
| different: |
| |
| ^((.)(?1)\2|.)$ |
| |
| This time, the recursing alternative is tried first, and continues to |
| recurse until it runs out of characters, at which point the recursion |
| fails. But this time we do have another alternative to try at the |
| higher level. That is the big difference: in the previous case the |
| remaining alternative is at a deeper recursion level, which PCRE cannot |
| use. |
| |
| To change the pattern so that it matches all palindromic strings, not |
| just those with an odd number of characters, it is tempting to change |
| the pattern to this: |
| |
| ^((.)(?1)\2|.?)$ |
| |
| Again, this works in Perl, but not in PCRE, and for the same reason. |
| When a deeper recursion has matched a single character, it cannot be |
| entered again in order to match an empty string. The solution is to |
| separate the two cases, and write out the odd and even cases as alter- |
| natives at the higher level: |
| |
| ^(?:((.)(?1)\2|)|((.)(?3)\4|.)) |
| |
| If you want to match typical palindromic phrases, the pattern has to |
| ignore all non-word characters, which can be done like this: |
| |
| ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$ |
| |
| If run with the PCRE_CASELESS option, this pattern matches phrases such |
| as "A man, a plan, a canal: Panama!" and it works well in both PCRE and |
| Perl. Note the use of the possessive quantifier *+ to avoid backtrack- |
| ing into sequences of non-word characters. Without this, PCRE takes a |
| great deal longer (ten times or more) to match typical phrases, and |
| Perl takes so long that you think it has gone into a loop. |
| |
| WARNING: The palindrome-matching patterns above work only if the sub- |
| ject string does not start with a palindrome that is shorter than the |
| entire string. For example, although "abcba" is correctly matched, if |
| the subject is "ababa", PCRE finds the palindrome "aba" at the start, |
| then fails at top level because the end of the string does not follow. |
| Once again, it cannot jump back into the recursion to try other alter- |
| natives, so the entire match fails. |
| |
| The second way in which PCRE and Perl differ in their recursion pro- |
| cessing is in the handling of captured values. In Perl, when a subpat- |
| tern is called recursively or as a subpattern (see the next section), |
| it has no access to any values that were captured outside the recur- |
| sion, whereas in PCRE these values can be referenced. Consider this |
| pattern: |
| |
| ^(.)(\1|a(?2)) |
| |
| In PCRE, this pattern matches "bab". The first capturing parentheses |
| match "b", then in the second group, when the back reference \1 fails |
| to match "b", the second alternative matches "a" and then recurses. In |
| the recursion, \1 does now match "b" and so the whole match succeeds. |
| In Perl, the pattern fails to match because inside the recursive call |
| \1 cannot access the externally set value. |
| |
| |
| SUBPATTERNS AS SUBROUTINES |
| |
| If the syntax for a recursive subpattern call (either by number or by |
| name) is used outside the parentheses to which it refers, it operates |
| like a subroutine in a programming language. The called subpattern may |
| be defined before or after the reference. A numbered reference can be |
| absolute or relative, as in these examples: |
| |
| (...(absolute)...)...(?2)... |
| (...(relative)...)...(?-1)... |
| (...(?+1)...(relative)... |
| |
| An earlier example pointed out that the pattern |
| |
| (sens|respons)e and \1ibility |
| |
| matches "sense and sensibility" and "response and responsibility", but |
| not "sense and responsibility". If instead the pattern |
| |
| (sens|respons)e and (?1)ibility |
| |
| is used, it does match "sense and responsibility" as well as the other |
| two strings. Another example is given in the discussion of DEFINE |
| above. |
| |
| All subroutine calls, whether recursive or not, are always treated as |
| atomic groups. That is, once a subroutine has matched some of the sub- |
| ject string, it is never re-entered, even if it contains untried alter- |
| natives and there is a subsequent matching failure. Any capturing |
| parentheses that are set during the subroutine call revert to their |
| previous values afterwards. |
| |
| Processing options such as case-independence are fixed when a subpat- |
| tern is defined, so if it is used as a subroutine, such options cannot |
| be changed for different calls. For example, consider this pattern: |
| |
| (abc)(?i:(?-1)) |
| |
| It matches "abcabc". It does not match "abcABC" because the change of |
| processing option does not affect the called subpattern. |
| |
| |
| ONIGURUMA SUBROUTINE SYNTAX |
| |
| For compatibility with Oniguruma, the non-Perl syntax \g followed by a |
| name or a number enclosed either in angle brackets or single quotes, is |
| an alternative syntax for referencing a subpattern as a subroutine, |
| possibly recursively. Here are two of the examples used above, rewrit- |
| ten using this syntax: |
| |
| (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) |
| (sens|respons)e and \g'1'ibility |
| |
| PCRE supports an extension to Oniguruma: if a number is preceded by a |
| plus or a minus sign it is taken as a relative reference. For example: |
| |
| (abc)(?i:\g<-1>) |
| |
| Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not |
| synonymous. The former is a back reference; the latter is a subroutine |
| call. |
| |
| |
| CALLOUTS |
| |
| Perl has a feature whereby using the sequence (?{...}) causes arbitrary |
| Perl code to be obeyed in the middle of matching a regular expression. |
| This makes it possible, amongst other things, to extract different sub- |
| strings that match the same pair of parentheses when there is a repeti- |
| tion. |
| |
| PCRE provides a similar feature, but of course it cannot obey arbitrary |
| Perl code. The feature is called "callout". The caller of PCRE provides |
| an external function by putting its entry point in the global variable |
| pcre_callout. By default, this variable contains NULL, which disables |
| all calling out. |
| |
| Within a regular expression, (?C) indicates the points at which the |
| external function is to be called. If you want to identify different |
| callout points, you can put a number less than 256 after the letter C. |
| The default value is zero. For example, this pattern has two callout |
| points: |
| |
| (?C1)abc(?C2)def |
| |
| If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are |
| automatically installed before each item in the pattern. They are all |
| numbered 255. |
| |
| During matching, when PCRE reaches a callout point (and pcre_callout is |
| set), the external function is called. It is provided with the number |
| of the callout, the position in the pattern, and, optionally, one item |
| of data originally supplied by the caller of pcre_exec(). The callout |
| function may cause matching to proceed, to backtrack, or to fail alto- |
| gether. A complete description of the interface to the callout function |
| is given in the pcrecallout documentation. |
| |
| |
| BACKTRACKING CONTROL |
| |
| Perl 5.10 introduced a number of "Special Backtracking Control Verbs", |
| which are described in the Perl documentation as "experimental and sub- |
| ject to change or removal in a future version of Perl". It goes on to |
| say: "Their usage in production code should be noted to avoid problems |
| during upgrades." The same remarks apply to the PCRE features described |
| in this section. |
| |
| Since these verbs are specifically related to backtracking, most of |
| them can be used only when the pattern is to be matched using |
| pcre_exec(), which uses a backtracking algorithm. With the exception of |
| (*FAIL), which behaves like a failing negative assertion, they cause an |
| error if encountered by pcre_dfa_exec(). |
| |
| If any of these verbs are used in an assertion or in a subpattern that |
| is called as a subroutine (whether or not recursively), their effect is |
| confined to that subpattern; it does not extend to the surrounding pat- |
| tern, with one exception: the name from a *(MARK), (*PRUNE), or (*THEN) |
| that is encountered in a successful positive assertion is passed back |
| when a match succeeds (compare capturing parentheses in assertions). |
| Note that such subpatterns are processed as anchored at the point where |
| they are tested. Note also that Perl's treatment of subroutines is dif- |
| ferent in some cases. |
| |
| The new verbs make use of what was previously invalid syntax: an open- |
| ing parenthesis followed by an asterisk. They are generally of the form |
| (*VERB) or (*VERB:NAME). Some may take either form, with differing be- |
| haviour, depending on whether or not an argument is present. A name is |
| any sequence of characters that does not include a closing parenthesis. |
| If the name is empty, that is, if the closing parenthesis immediately |
| follows the colon, the effect is as if the colon were not there. Any |
| number of these verbs may occur in a pattern. |
| |
| PCRE contains some optimizations that are used to speed up matching by |
| running some checks at the start of each match attempt. For example, it |
| may know the minimum length of matching subject, or that a particular |
| character must be present. When one of these optimizations suppresses |
| the running of a match, any included backtracking verbs will not, of |
| course, be processed. You can suppress the start-of-match optimizations |
| by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com- |
| pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT). |
| |
| Experiments with Perl suggest that it too has similar optimizations, |
| sometimes leading to anomalous results. |
| |
| Verbs that act immediately |
| |
| The following verbs act as soon as they are encountered. They may not |
| be followed by a name. |
| |
| (*ACCEPT) |
| |
| This verb causes the match to end successfully, skipping the remainder |
| of the pattern. However, when it is inside a subpattern that is called |
| as a subroutine, only that subpattern is ended successfully. Matching |
| then continues at the outer level. If (*ACCEPT) is inside capturing |
| parentheses, the data so far is captured. For example: |
| |
| A((?:A|B(*ACCEPT)|C)D) |
| |
| This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap- |
| tured by the outer parentheses. |
| |
| (*FAIL) or (*F) |
| |
| This verb causes a matching failure, forcing backtracking to occur. It |
| is equivalent to (?!) but easier to read. The Perl documentation notes |
| that it is probably useful only when combined with (?{}) or (??{}). |
| Those are, of course, Perl features that are not present in PCRE. The |
| nearest equivalent is the callout feature, as for example in this pat- |
| tern: |
| |
| a+(?C)(*FAIL) |
| |
| A match with the string "aaaa" always fails, but the callout is taken |
| before each backtrack happens (in this example, 10 times). |
| |
| Recording which path was taken |
| |
| There is one verb whose main purpose is to track how a match was |
| arrived at, though it also has a secondary use in conjunction with |
| advancing the match starting point (see (*SKIP) below). |
| |
| (*MARK:NAME) or (*:NAME) |
| |
| A name is always required with this verb. There may be as many |
| instances of (*MARK) as you like in a pattern, and their names do not |
| have to be unique. |
| |
| When a match succeeds, the name of the last-encountered (*MARK) on the |
| matching path is passed back to the caller via the pcre_extra data |
| structure, as described in the section on pcre_extra in the pcreapi |
| documentation. Here is an example of pcretest output, where the /K mod- |
| ifier requests the retrieval and outputting of (*MARK) data: |
| |
| re> /X(*MARK:A)Y|X(*MARK:B)Z/K |
| data> XY |
| 0: XY |
| MK: A |
| XZ |
| 0: XZ |
| MK: B |
| |
| The (*MARK) name is tagged with "MK:" in this output, and in this exam- |
| ple it indicates which of the two alternatives matched. This is a more |
| efficient way of obtaining this information than putting each alterna- |
| tive in its own capturing parentheses. |
| |
| If (*MARK) is encountered in a positive assertion, its name is recorded |
| and passed back if it is the last-encountered. This does not happen for |
| negative assertions. |
| |
| After a partial match or a failed match, the name of the last encoun- |
| tered (*MARK) in the entire match process is returned. For example: |
| |
| re> /X(*MARK:A)Y|X(*MARK:B)Z/K |
| data> XP |
| No match, mark = B |
| |
| Note that in this unanchored example the mark is retained from the |
| match attempt that started at the letter "X". Subsequent match attempts |
| starting at "P" and then with an empty string do not get as far as the |
| (*MARK) item, but nevertheless do not reset it. |
| |
| Verbs that act after backtracking |
| |
| The following verbs do nothing when they are encountered. Matching con- |
| tinues with what follows, but if there is no subsequent match, causing |
| a backtrack to the verb, a failure is forced. That is, backtracking |
| cannot pass to the left of the verb. However, when one of these verbs |
| appears inside an atomic group, its effect is confined to that group, |
| because once the group has been matched, there is never any backtrack- |
| ing into it. In this situation, backtracking can "jump back" to the |
| left of the entire atomic group. (Remember also, as stated above, that |
| this localization also applies in subroutine calls and assertions.) |
| |
| These verbs differ in exactly what kind of failure occurs when back- |
| tracking reaches them. |
| |
| (*COMMIT) |
| |
| This verb, which may not be followed by a name, causes the whole match |
| to fail outright if the rest of the pattern does not match. Even if the |
| pattern is unanchored, no further attempts to find a match by advancing |
| the starting point take place. Once (*COMMIT) has been passed, |
| pcre_exec() is committed to finding a match at the current starting |
| point, or not at all. For example: |
| |
| a+(*COMMIT)b |
| |
| This matches "xxaab" but not "aacaab". It can be thought of as a kind |
| of dynamic anchor, or "I've started, so I must finish." The name of the |
| most recently passed (*MARK) in the path is passed back when (*COMMIT) |
| forces a match failure. |
| |
| Note that (*COMMIT) at the start of a pattern is not the same as an |
| anchor, unless PCRE's start-of-match optimizations are turned off, as |
| shown in this pcretest example: |
| |
| re> /(*COMMIT)abc/ |
| data> xyzabc |
| 0: abc |
| xyzabc\Y |
| No match |
| |
| PCRE knows that any match must start with "a", so the optimization |
| skips along the subject to "a" before running the first match attempt, |
| which succeeds. When the optimization is disabled by the \Y escape in |
| the second subject, the match starts at "x" and so the (*COMMIT) causes |
| it to fail without trying any other starting points. |
| |
| (*PRUNE) or (*PRUNE:NAME) |
| |
| This verb causes the match to fail at the current starting position in |
| the subject if the rest of the pattern does not match. If the pattern |
| is unanchored, the normal "bumpalong" advance to the next starting |
| character then happens. Backtracking can occur as usual to the left of |
| (*PRUNE), before it is reached, or when matching to the right of |
| (*PRUNE), but if there is no match to the right, backtracking cannot |
| cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alter- |
| native to an atomic group or possessive quantifier, but there are some |
| uses of (*PRUNE) that cannot be expressed in any other way. The behav- |
| iour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an |
| anchored pattern (*PRUNE) has the same effect as (*COMMIT). |
| |
| (*SKIP) |
| |
| This verb, when given without a name, is like (*PRUNE), except that if |
| the pattern is unanchored, the "bumpalong" advance is not to the next |
| character, but to the position in the subject where (*SKIP) was encoun- |
| tered. (*SKIP) signifies that whatever text was matched leading up to |
| it cannot be part of a successful match. Consider: |
| |
| a+(*SKIP)b |
| |
| If the subject is "aaaac...", after the first match attempt fails |
| (starting at the first character in the string), the starting point |
| skips on to start the next attempt at "c". Note that a possessive quan- |
| tifer does not have the same effect as this example; although it would |
| suppress backtracking during the first match attempt, the second |
| attempt would start at the second character instead of skipping on to |
| "c". |
| |
| (*SKIP:NAME) |
| |
| When (*SKIP) has an associated name, its behaviour is modified. If the |
| following pattern fails to match, the previous path through the pattern |
| is searched for the most recent (*MARK) that has the same name. If one |
| is found, the "bumpalong" advance is to the subject position that cor- |
| responds to that (*MARK) instead of to where (*SKIP) was encountered. |
| If no (*MARK) with a matching name is found, the (*SKIP) is ignored. |
| |
| (*THEN) or (*THEN:NAME) |
| |
| This verb causes a skip to the next innermost alternative if the rest |
| of the pattern does not match. That is, it cancels pending backtrack- |
| ing, but only within the current alternative. Its name comes from the |
| observation that it can be used for a pattern-based if-then-else block: |
| |
| ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... |
| |
| If the COND1 pattern matches, FOO is tried (and possibly further items |
| after the end of the group if FOO succeeds); on failure, the matcher |
| skips to the second alternative and tries COND2, without backtracking |
| into COND1. The behaviour of (*THEN:NAME) is exactly the same as |
| (*MARK:NAME)(*THEN). If (*THEN) is not inside an alternation, it acts |
| like (*PRUNE). |
| |
| Note that a subpattern that does not contain a | character is just a |
| part of the enclosing alternative; it is not a nested alternation with |
| only one alternative. The effect of (*THEN) extends beyond such a sub- |
| pattern to the enclosing alternative. Consider this pattern, where A, |
| B, etc. are complex pattern fragments that do not contain any | charac- |
| ters at this level: |
| |
| A (B(*THEN)C) | D |
| |
| If A and B are matched, but there is a failure in C, matching does not |
| backtrack into A; instead it moves to the next alternative, that is, D. |
| However, if the subpattern containing (*THEN) is given an alternative, |
| it behaves differently: |
| |
| A (B(*THEN)C | (*FAIL)) | D |
| |
| The effect of (*THEN) is now confined to the inner subpattern. After a |
| failure in C, matching moves to (*FAIL), which causes the whole subpat- |
| tern to fail because there are no more alternatives to try. In this |
| case, matching does now backtrack into A. |
| |
| Note also that a conditional subpattern is not considered as having two |
| alternatives, because only one is ever used. In other words, the | |
| character in a conditional subpattern has a different meaning. Ignoring |
| white space, consider: |
| |
| ^.*? (?(?=a) a | b(*THEN)c ) |
| |
| If the subject is "ba", this pattern does not match. Because .*? is |
| ungreedy, it initially matches zero characters. The condition (?=a) |
| then fails, the character "b" is matched, but "c" is not. At this |
| point, matching does not backtrack to .*? as might perhaps be expected |
| from the presence of the | character. The conditional subpattern is |
| part of the single alternative that comprises the whole pattern, and so |
| the match fails. (If there was a backtrack into .*?, allowing it to |
| match "b", the match would succeed.) |
| |
| The verbs just described provide four different "strengths" of control |
| when subsequent matching fails. (*THEN) is the weakest, carrying on the |
| match at the next alternative. (*PRUNE) comes next, failing the match |
| at the current starting position, but allowing an advance to the next |
| character (for an unanchored pattern). (*SKIP) is similar, except that |
| the advance may be more than one character. (*COMMIT) is the strongest, |
| causing the entire match to fail. |
| |
| If more than one such verb is present in a pattern, the "strongest" one |
| wins. For example, consider this pattern, where A, B, etc. are complex |
| pattern fragments: |
| |
| (A(*COMMIT)B(*THEN)C|D) |
| |
| Once A has matched, PCRE is committed to this match, at the current |
| starting position. If subsequently B matches, but C does not, the nor- |
| mal (*THEN) action of trying the next alternative (that is, D) does not |
| happen because (*COMMIT) overrides. |
| |
| |
| SEE ALSO |
| |
| pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3). |
| |
| |
| AUTHOR |
| |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| |
| REVISION |
| |
| Last updated: 29 November 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| |
| |
| PCRESYNTAX(3) PCRESYNTAX(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions |
| |
| |
| PCRE REGULAR EXPRESSION SYNTAX SUMMARY |
| |
| The full syntax and semantics of the regular expressions that are sup- |
| ported by PCRE are described in the pcrepattern documentation. This |
| document contains just a quick-reference summary of the syntax. |
| |
| |
| QUOTING |
| |
| \x where x is non-alphanumeric is a literal x |
| \Q...\E treat enclosed characters as literal |
| |
| |
| CHARACTERS |
| |
| \a alarm, that is, the BEL character (hex 07) |
| \cx "control-x", where x is any ASCII character |
| \e escape (hex 1B) |
| \f formfeed (hex 0C) |
| \n newline (hex 0A) |
| \r carriage return (hex 0D) |
| \t tab (hex 09) |
| \ddd character with octal code ddd, or backreference |
| \xhh character with hex code hh |
| \x{hhh..} character with hex code hhh.. |
| |
| |
| CHARACTER TYPES |
| |
| . any character except newline; |
| in dotall mode, any character whatsoever |
| \C one byte, even in UTF-8 mode (best avoided) |
| \d a decimal digit |
| \D a character that is not a decimal digit |
| \h a horizontal whitespace character |
| \H a character that is not a horizontal whitespace character |
| \N a character that is not a newline |
| \p{xx} a character with the xx property |
| \P{xx} a character without the xx property |
| \R a newline sequence |
| \s a whitespace character |
| \S a character that is not a whitespace character |
| \v a vertical whitespace character |
| \V a character that is not a vertical whitespace character |
| \w a "word" character |
| \W a "non-word" character |
| \X an extended Unicode sequence |
| |
| In PCRE, by default, \d, \D, \s, \S, \w, and \W recognize only ASCII |
| characters, even in UTF-8 mode. However, this can be changed by setting |
| the PCRE_UCP option. |
| |
| |
| GENERAL CATEGORY PROPERTIES FOR \p and \P |
| |
| C Other |
| Cc Control |
| Cf Format |
| Cn Unassigned |
| Co Private use |
| Cs Surrogate |
| |
| L Letter |
| Ll Lower case letter |
| Lm Modifier letter |
| Lo Other letter |
| Lt Title case letter |
| Lu Upper case letter |
| L& Ll, Lu, or Lt |
| |
| M Mark |
| Mc Spacing mark |
| Me Enclosing mark |
| Mn Non-spacing mark |
| |
| N Number |
| Nd Decimal number |
| Nl Letter number |
| No Other number |
| |
| P Punctuation |
| Pc Connector punctuation |
| Pd Dash punctuation |
| Pe Close punctuation |
| Pf Final punctuation |
| Pi Initial punctuation |
| Po Other punctuation |
| Ps Open punctuation |
| |
| S Symbol |
| Sc Currency symbol |
| Sk Modifier symbol |
| Sm Mathematical symbol |
| So Other symbol |
| |
| Z Separator |
| Zl Line separator |
| Zp Paragraph separator |
| Zs Space separator |
| |
| |
| PCRE SPECIAL CATEGORY PROPERTIES FOR \p and \P |
| |
| Xan Alphanumeric: union of properties L and N |
| Xps POSIX space: property Z or tab, NL, VT, FF, CR |
| Xsp Perl space: property Z or tab, NL, FF, CR |
| Xwd Perl word: property Xan or underscore |
| |
| |
| SCRIPT NAMES FOR \p AND \P |
| |
| Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille, |
| Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common, |
| Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyp- |
| tian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek, |
| Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Impe- |
| rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian, |
| Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao, |
| Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam, |
| Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic, |
| Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya, |
| Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian, |
| Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, |
| Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, |
| Ugaritic, Vai, Yi. |
| |
| |
| CHARACTER CLASSES |
| |
| [...] positive character class |
| [^...] negative character class |
| [x-y] range (can be used for hex characters) |
| [[:xxx:]] positive POSIX named set |
| [[:^xxx:]] negative POSIX named set |
| |
| alnum alphanumeric |
| alpha alphabetic |
| ascii 0-127 |
| blank space or tab |
| cntrl control character |
| digit decimal digit |
| graph printing, excluding space |
| lower lower case letter |
| print printing, including space |
| punct printing, excluding alphanumeric |
| space whitespace |
| upper upper case letter |
| word same as \w |
| xdigit hexadecimal digit |
| |
| In PCRE, POSIX character set names recognize only ASCII characters by |
| default, but some of them use Unicode properties if PCRE_UCP is set. |
| You can use \Q...\E inside a character class. |
| |
| |
| QUANTIFIERS |
| |
| ? 0 or 1, greedy |
| ?+ 0 or 1, possessive |
| ?? 0 or 1, lazy |
| * 0 or more, greedy |
| *+ 0 or more, possessive |
| *? 0 or more, lazy |
| + 1 or more, greedy |
| ++ 1 or more, possessive |
| +? 1 or more, lazy |
| {n} exactly n |
| {n,m} at least n, no more than m, greedy |
| {n,m}+ at least n, no more than m, possessive |
| {n,m}? at least n, no more than m, lazy |
| {n,} n or more, greedy |
| {n,}+ n or more, possessive |
| {n,}? n or more, lazy |
| |
| |
| ANCHORS AND SIMPLE ASSERTIONS |
| |
| \b word boundary |
| \B not a word boundary |
| ^ start of subject |
| also after internal newline in multiline mode |
| \A start of subject |
| $ end of subject |
| also before newline at end of subject |
| also before internal newline in multiline mode |
| \Z end of subject |
| also before newline at end of subject |
| \z end of subject |
| \G first matching position in subject |
| |
| |
| MATCH POINT RESET |
| |
| \K reset start of match |
| |
| |
| ALTERNATION |
| |
| expr|expr|expr... |
| |
| |
| CAPTURING |
| |
| (...) capturing group |
| (?<name>...) named capturing group (Perl) |
| (?'name'...) named capturing group (Perl) |
| (?P<name>...) named capturing group (Python) |
| (?:...) non-capturing group |
| (?|...) non-capturing group; reset group numbers for |
| capturing groups in each alternative |
| |
| |
| ATOMIC GROUPS |
| |
| (?>...) atomic, non-capturing group |
| |
| |
| COMMENT |
| |
| (?#....) comment (not nestable) |
| |
| |
| OPTION SETTING |
| |
| (?i) caseless |
| (?J) allow duplicate names |
| (?m) multiline |
| (?s) single line (dotall) |
| (?U) default ungreedy (lazy) |
| (?x) extended (ignore white space) |
| (?-...) unset option(s) |
| |
| The following are recognized only at the start of a pattern or after |
| one of the newline-setting options with similar syntax: |
| |
| (*NO_START_OPT) no start-match optimization (PCRE_NO_START_OPTIMIZE) |
| (*UTF8) set UTF-8 mode (PCRE_UTF8) |
| (*UCP) set PCRE_UCP (use Unicode properties for \d etc) |
| |
| |
| LOOKAHEAD AND LOOKBEHIND ASSERTIONS |
| |
| (?=...) positive look ahead |
| (?!...) negative look ahead |
| (?<=...) positive look behind |
| (?<!...) negative look behind |
| |
| Each top-level branch of a look behind must be of a fixed length. |
| |
| |
| BACKREFERENCES |
| |
| \n reference by number (can be ambiguous) |
| \gn reference by number |
| \g{n} reference by number |
| \g{-n} relative reference by number |
| \k<name> reference by name (Perl) |
| \k'name' reference by name (Perl) |
| \g{name} reference by name (Perl) |
| \k{name} reference by name (.NET) |
| (?P=name) reference by name (Python) |
| |
| |
| SUBROUTINE REFERENCES (POSSIBLY RECURSIVE) |
| |
| (?R) recurse whole pattern |
| (?n) call subpattern by absolute number |
| (?+n) call subpattern by relative number |
| (?-n) call subpattern by relative number |
| (?&name) call subpattern by name (Perl) |
| (?P>name) call subpattern by name (Python) |
| \g<name> call subpattern by name (Oniguruma) |
| \g'name' call subpattern by name (Oniguruma) |
| \g<n> call subpattern by absolute number (Oniguruma) |
| \g'n' call subpattern by absolute number (Oniguruma) |
| \g<+n> call subpattern by relative number (PCRE extension) |
| \g'+n' call subpattern by relative number (PCRE extension) |
| \g<-n> call subpattern by relative number (PCRE extension) |
| \g'-n' call subpattern by relative number (PCRE extension) |
| |
| |
| CONDITIONAL PATTERNS |
| |
| (?(condition)yes-pattern) |
| (?(condition)yes-pattern|no-pattern) |
| |
| (?(n)... absolute reference condition |
| (?(+n)... relative reference condition |
| (?(-n)... relative reference condition |
| (?(<name>)... named reference condition (Perl) |
| (?('name')... named reference condition (Perl) |
| (?(name)... named reference condition (PCRE) |
| (?(R)... overall recursion condition |
| (?(Rn)... specific group recursion condition |
| (?(R&name)... specific recursion condition |
| (?(DEFINE)... define subpattern for reference |
| (?(assert)... assertion condition |
| |
| |
| BACKTRACKING CONTROL |
| |
| The following act immediately they are reached: |
| |
| (*ACCEPT) force successful match |
| (*FAIL) force backtrack; synonym (*F) |
| |
| The following act only when a subsequent match failure causes a back- |
| track to reach them. They all force a match failure, but they differ in |
| what happens afterwards. Those that advance the start-of-match point do |
| so only if the pattern is not anchored. |
| |
| (*COMMIT) overall failure, no advance of starting point |
| (*PRUNE) advance to next starting character |
| (*SKIP) advance start to current matching position |
| (*THEN) local failure, backtrack to next alternation |
| |
| |
| NEWLINE CONVENTIONS |
| |
| These are recognized only at the very start of the pattern or after a |
| (*BSR_...) or (*UTF8) or (*UCP) option. |
| |
| (*CR) carriage return only |
| (*LF) linefeed only |
| (*CRLF) carriage return followed by linefeed |
| (*ANYCRLF) all three of the above |
| (*ANY) any Unicode newline sequence |
| |
| |
| WHAT \R MATCHES |
| |
| These are recognized only at the very start of the pattern or after a |
| (*...) option that sets the newline convention or UTF-8 or UCP mode. |
| |
| (*BSR_ANYCRLF) CR, LF, or CRLF |
| (*BSR_UNICODE) any Unicode newline sequence |
| |
| |
| CALLOUTS |
| |
| (?C) callout |
| (?Cn) callout with data n |
| |
| |
| SEE ALSO |
| |
| pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3). |
| |
| |
| AUTHOR |
| |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| |
| REVISION |
| |
| Last updated: 21 November 2010 |
| Copyright (c) 1997-2010 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| |
| |
| PCREUNICODE(3) PCREUNICODE(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions |
| |
| |
| UTF-8 AND UNICODE PROPERTY SUPPORT |
| |
| In order process UTF-8 strings, you must build PCRE to include UTF-8 |
| support in the code, and, in addition, you must call pcre_compile() |
| with the PCRE_UTF8 option flag, or the pattern must start with the |
| sequence (*UTF8). When either of these is the case, both the pattern |
| and any subject strings that are matched against it are treated as |
| UTF-8 strings instead of strings of 1-byte characters. PCRE does not |
| support any other formats (in particular, it does not support UTF-16). |
| |
| If you compile PCRE with UTF-8 support, but do not use it at run time, |
| the library will be a bit bigger, but the additional run time overhead |
| is limited to testing the PCRE_UTF8 flag occasionally, so should not be |
| very big. |
| |
| If PCRE is built with Unicode character property support (which implies |
| UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup- |
| ported. The available properties that can be tested are limited to the |
| general category properties such as Lu for an upper case letter or Nd |
| for a decimal number, the Unicode script names such as Arabic or Han, |
| and the derived properties Any and L&. A full list is given in the |
| pcrepattern documentation. Only the short names for properties are sup- |
| ported. For example, \p{L} matches a letter. Its Perl synonym, \p{Let- |
| ter}, is not supported. Furthermore, in Perl, many properties may |
| optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE |
| does not support this. |
| |
| Validity of UTF-8 strings |
| |
| When you set the PCRE_UTF8 flag, the strings passed as patterns and |
| subjects are (by default) checked for validity on entry to the relevant |
| functions. From release 7.3 of PCRE, the check is according the rules |
| of RFC 3629, which are themselves derived from the Unicode specifica- |
| tion. Earlier releases of PCRE followed the rules of RFC 2279, which |
| allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current |
| check allows only values in the range U+0 to U+10FFFF, excluding U+D800 |
| to U+DFFF. |
| |
| The excluded code points are the "Low Surrogate Area" of Unicode, of |
| which the Unicode Standard says this: "The Low Surrogate Area does not |
| contain any character assignments, consequently no character code |
| charts or namelists are provided for this area. Surrogates are reserved |
| for use with UTF-16 and then must be used in pairs." The code points |
| that are encoded by UTF-16 pairs are available as independent code |
| points in the UTF-8 encoding. (In other words, the whole surrogate |
| thing is a fudge for UTF-16 which unfortunately messes up UTF-8.) |
| |
| If an invalid UTF-8 string is passed to PCRE, an error return is given. |
| At compile time, the only additional information is the offset to the |
| first byte of the failing character. The runtime functions pcre_exec() |
| and pcre_dfa_exec() also pass back this information, as well as a more |
| detailed reason code if the caller has provided memory in which to do |
| this. |
| |
| In some situations, you may already know that your strings are valid, |
| and therefore want to skip these checks in order to improve perfor- |
| mance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or at run |
| time, PCRE assumes that the pattern or subject it is given (respec- |
| tively) contains only valid UTF-8 codes. In this case, it does not |
| diagnose an invalid UTF-8 string. |
| |
| If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, |
| what happens depends on why the string is invalid. If the string con- |
| forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a |
| string of characters in the range 0 to 0x7FFFFFFF by pcre_dfa_exec() |
| and the interpreted version of pcre_exec(). In other words, apart from |
| the initial validity test, these functions (when in UTF-8 mode) handle |
| strings according to the more liberal rules of RFC 2279. However, the |
| just-in-time (JIT) optimization for pcre_exec() supports only RFC 3629. |
| If you are using JIT optimization, or if the string does not even con- |
| form to RFC 2279, the result is undefined. Your program may crash. |
| |
| If you want to process strings of values in the full range 0 to |
| 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can |
| set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in |
| this situation, you will have to apply your own validity check, and |
| avoid the use of JIT optimization. |
| |
| General comments about UTF-8 mode |
| |
| 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a |
| two-byte UTF-8 character if the value is greater than 127. |
| |
| 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8 |
| characters for values greater than \177. |
| |
| 3. Repeat quantifiers apply to complete UTF-8 characters, not to indi- |
| vidual bytes, for example: \x{100}{3}. |
| |
| 4. The dot metacharacter matches one UTF-8 character instead of a sin- |
| gle byte. |
| |
| 5. The escape sequence \C can be used to match a single byte in UTF-8 |
| mode, but its use can lead to some strange effects because it breaks up |
| multibyte characters (see the description of \C in the pcrepattern doc- |
| umentation). The use of \C is not supported in the alternative matching |
| function pcre_dfa_exec(), nor is it supported in UTF-8 mode by the JIT |
| optimization of pcre_exec(). If JIT optimization is requested for a |
| UTF-8 pattern that contains \C, it will not succeed, and so the match- |
| ing will be carried out by the normal interpretive function. |
| |
| 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly |
| test characters of any code value, but, by default, the characters that |
| PCRE recognizes as digits, spaces, or word characters remain the same |
| set as before, all with values less than 256. This remains true even |
| when PCRE is built to include Unicode property support, because to do |
| otherwise would slow down PCRE in many common cases. Note in particular |
| that this applies to \b and \B, because they are defined in terms of \w |
| and \W. If you really want to test for a wider sense of, say, "digit", |
| you can use explicit Unicode property tests such as \p{Nd}. Alterna- |
| tively, if you set the PCRE_UCP option, the way that the character |
| escapes work is changed so that Unicode properties are used to deter- |
| mine which characters match. There are more details in the section on |
| generic character types in the pcrepattern documentation. |
| |
| 7. Similarly, characters that match the POSIX named character classes |
| are all low-valued characters, unless the PCRE_UCP option is set. |
| |
| 8. However, the horizontal and vertical whitespace matching escapes |
| (\h, \H, \v, and \V) do match all the appropriate Unicode characters, |
| whether or not PCRE_UCP is set. |
| |
| 9. Case-insensitive matching applies only to characters whose values |
| are less than 128, unless PCRE is built with Unicode property support. |
| Even when Unicode property support is available, PCRE still uses its |
| own character tables when checking the case of low-valued characters, |
| so as not to degrade performance. The Unicode property information is |
| used only for characters with higher values. Furthermore, PCRE supports |
| case-insensitive matching only when there is a one-to-one mapping |
| between a letter's cases. There are a small number of many-to-one map- |
| pings in Unicode; these are not supported by PCRE. |
| |
| |
| AUTHOR |
| |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| |
| REVISION |
| |
| Last updated: 19 October 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| |
| |
| PCREJIT(3) PCREJIT(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions |
| |
| |
| PCRE JUST-IN-TIME COMPILER SUPPORT |
| |
| Just-in-time compiling is a heavyweight optimization that can greatly |
| speed up pattern matching. However, it comes at the cost of extra pro- |
| cessing before the match is performed. Therefore, it is of most benefit |
| when the same pattern is going to be matched many times. This does not |
| necessarily mean many calls of pcre_exec(); if the pattern is not |
| anchored, matching attempts may take place many times at various posi- |
| tions in the subject, even for a single call to pcre_exec(). If the |
| subject string is very long, it may still pay to use JIT for one-off |
| matches. |
| |
| JIT support applies only to the traditional matching function, |
| pcre_exec(). It does not apply when pcre_dfa_exec() is being used. The |
| code for this support was written by Zoltan Herczeg. |
| |
| |
| AVAILABILITY OF JIT SUPPORT |
| |
| JIT support is an optional feature of PCRE. The "configure" option |
| --enable-jit (or equivalent CMake option) must be set when PCRE is |
| built if you want to use JIT. The support is limited to the following |
| hardware platforms: |
| |
| ARM v5, v7, and Thumb2 |
| Intel x86 32-bit and 64-bit |
| MIPS 32-bit |
| Power PC 32-bit and 64-bit (experimental) |
| |
| The Power PC support is designated as experimental because it has not |
| been fully tested. If --enable-jit is set on an unsupported platform, |
| compilation fails. |
| |
| A program that is linked with PCRE 8.20 or later can tell if JIT sup- |
| port is available by calling pcre_config() with the PCRE_CONFIG_JIT |
| option. The result is 1 when JIT is available, and 0 otherwise. How- |
| ever, a simple program does not need to check this in order to use JIT. |
| The API is implemented in a way that falls back to the ordinary PCRE |
| code if JIT is not available. |
| |
| If your program may sometimes be linked with versions of PCRE that are |
| older than 8.20, but you want to use JIT when it is available, you can |
| test the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT |
| macro such as PCRE_CONFIG_JIT, for compile-time control of your code. |
| |
| |
| SIMPLE USE OF JIT |
| |
| You have to do two things to make use of the JIT support in the sim- |
| plest way: |
| |
| (1) Call pcre_study() with the PCRE_STUDY_JIT_COMPILE option for |
| each compiled pattern, and pass the resulting pcre_extra block to |
| pcre_exec(). |
| |
| (2) Use pcre_free_study() to free the pcre_extra block when it is |
| no longer needed instead of just freeing it yourself. This |
| ensures that any JIT data is also freed. |
| |
| For a program that may be linked with pre-8.20 versions of PCRE, you |
| can insert |
| |
| #ifndef PCRE_STUDY_JIT_COMPILE |
| #define PCRE_STUDY_JIT_COMPILE 0 |
| #endif |
| |
| so that no option is passed to pcre_study(), and then use something |
| like this to free the study data: |
| |
| #ifdef PCRE_CONFIG_JIT |
| pcre_free_study(study_ptr); |
| #else |
| pcre_free(study_ptr); |
| #endif |
| |
| In some circumstances you may need to call additional functions. These |
| are described in the section entitled "Controlling the JIT stack" |
| below. |
| |
| If JIT support is not available, PCRE_STUDY_JIT_COMPILE is ignored, and |
| no JIT data is set up. Otherwise, the compiled pattern is passed to the |
| JIT compiler, which turns it into machine code that executes much |
| faster than the normal interpretive code. When pcre_exec() is passed a |
| pcre_extra block containing a pointer to JIT code, it obeys that |
| instead of the normal code. The result is identical, but the code runs |
| much faster. |
| |
| There are some pcre_exec() options that are not supported for JIT exe- |
| cution. There are also some pattern items that JIT cannot handle. |
| Details are given below. In both cases, execution automatically falls |
| back to the interpretive code. |
| |
| If the JIT compiler finds an unsupported item, no JIT data is gener- |
| ated. You can find out if JIT execution is available after studying a |
| pattern by calling pcre_fullinfo() with the PCRE_INFO_JIT option. A |
| result of 1 means that JIT compilation was successful. A result of 0 |
| means that JIT support is not available, or the pattern was not studied |
| with PCRE_STUDY_JIT_COMPILE, or the JIT compiler was not able to handle |
| the pattern. |
| |
| Once a pattern has been studied, with or without JIT, it can be used as |
| many times as you like for matching different subject strings. |
| |
| |
| UNSUPPORTED OPTIONS AND PATTERN ITEMS |
| |
| The only pcre_exec() options that are supported for JIT execution are |
| PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and |
| PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not |
| supported. |
| |
| The unsupported pattern items are: |
| |
| \C match a single byte; not supported in UTF-8 mode |
| (?Cn) callouts |
| (*COMMIT) ) |
| (*MARK) ) |
| (*PRUNE) ) the backtracking control verbs |
| (*SKIP) ) |
| (*THEN) ) |
| |
| Support for some of these may be added in future. |
| |
| |
| RETURN VALUES FROM JIT EXECUTION |
| |
| When a pattern is matched using JIT execution, the return values are |
| the same as those given by the interpretive pcre_exec() code, with the |
| addition of one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means |
| that the memory used for the JIT stack was insufficient. See "Control- |
| ling the JIT stack" below for a discussion of JIT stack usage. For com- |
| patibility with the interpretive pcre_exec() code, no more than two- |
| thirds of the ovector argument is used for passing back captured sub- |
| strings. |
| |
| The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if |
| searching a very large pattern tree goes on for too long, as it is in |
| the same circumstance when JIT is not used, but the details of exactly |
| what is counted are not the same. The PCRE_ERROR_RECURSIONLIMIT error |
| code is never returned by JIT execution. |
| |
| |
| SAVING AND RESTORING COMPILED PATTERNS |
| |
| The code that is generated by the JIT compiler is architecture-spe- |
| cific, and is also position dependent. For those reasons it cannot be |
| saved (in a file or database) and restored later like the bytecode and |
| other data of a compiled pattern. Saving and restoring compiled pat- |
| terns is not something many people do. More detail about this facility |
| is given in the pcreprecompile documentation. It should be possible to |
| run pcre_study() on a saved and restored pattern, and thereby recreate |
| the JIT data, but because JIT compilation uses significant resources, |
| it is probably not worth doing this; you might as well recompile the |
| original pattern. |
| |
| |
| CONTROLLING THE JIT STACK |
| |
| When the compiled JIT code runs, it needs a block of memory to use as a |
| stack. By default, it uses 32K on the machine stack. However, some |
| large or complicated patterns need more than this. The error |
| PCRE_ERROR_JIT_STACKLIMIT is given when there is not enough stack. |
| Three functions are provided for managing blocks of memory for use as |
| JIT stacks. There is further discussion about the use of JIT stacks in |
| the section entitled "JIT stack FAQ" below. |
| |
| The pcre_jit_stack_alloc() function creates a JIT stack. Its arguments |
| are a starting size and a maximum size, and it returns a pointer to an |
| opaque structure of type pcre_jit_stack, or NULL if there is an error. |
| The pcre_jit_stack_free() function can be used to free a stack that is |
| no longer needed. (For the technically minded: the address space is |
| allocated by mmap or VirtualAlloc.) |
| |
| JIT uses far less memory for recursion than the interpretive code, and |
| a maximum stack size of 512K to 1M should be more than enough for any |
| pattern. |
| |
| The pcre_assign_jit_stack() function specifies which stack JIT code |
| should use. Its arguments are as follows: |
| |
| pcre_extra *extra |
| pcre_jit_callback callback |
| void *data |
| |
| The extra argument must be the result of studying a pattern with |
| PCRE_STUDY_JIT_COMPILE. There are three cases for the values of the |
| other two options: |
| |
| (1) If callback is NULL and data is NULL, an internal 32K block |
| on the machine stack is used. |
| |
| (2) If callback is NULL and data is not NULL, data must be |
| a valid JIT stack, the result of calling pcre_jit_stack_alloc(). |
| |
| (3) If callback not NULL, it must point to a function that is called |
| with data as an argument at the start of matching, in order to |
| set up a JIT stack. If the result is NULL, the internal 32K stack |
| is used; otherwise the return value must be a valid JIT stack, |
| the result of calling pcre_jit_stack_alloc(). |
| |
| You may safely assign the same JIT stack to more than one pattern, as |
| long as they are all matched sequentially in the same thread. In a mul- |
| tithread application, each thread must use its own JIT stack. |
| |
| Strictly speaking, even more is allowed. You can assign the same stack |
| to any number of patterns as long as they are not used for matching by |
| multiple threads at the same time. For example, you can assign the same |
| stack to all compiled patterns, and use a global mutex in the callback |
| to wait until the stack is available for use. However, this is an inef- |
| ficient solution, and not recommended. |
| |
| This is a suggestion for how a typical multithreaded program might |
| operate: |
| |
| During thread initalization |
| thread_local_var = pcre_jit_stack_alloc(...) |
| |
| During thread exit |
| pcre_jit_stack_free(thread_local_var) |
| |
| Use a one-line callback function |
| return thread_local_var |
| |
| All the functions described in this section do nothing if JIT is not |
| available, and pcre_assign_jit_stack() does nothing unless the extra |
| argument is non-NULL and points to a pcre_extra block that is the |
| result of a successful study with PCRE_STUDY_JIT_COMPILE. |
| |
| |
| JIT STACK FAQ |
| |
| (1) Why do we need JIT stacks? |
| |
| PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack |
| where the local data of the current node is pushed before checking its |
| child nodes. Allocating real machine stack on some platforms is diffi- |
| cult. For example, the stack chain needs to be updated every time if we |
| extend the stack on PowerPC. Although it is possible, its updating |
| time overhead decreases performance. So we do the recursion in memory. |
| |
| (2) Why don't we simply allocate blocks of memory with malloc()? |
| |
| Modern operating systems have a nice feature: they can reserve an |
| address space instead of allocating memory. We can safely allocate mem- |
| ory pages inside this address space, so the stack could grow without |
| moving memory data (this is important because of pointers). Thus we can |
| allocate 1M address space, and use only a single memory page (usually |
| 4K) if that is enough. However, we can still grow up to 1M anytime if |
| needed. |
| |
| (3) Who "owns" a JIT stack? |
| |
| The owner of the stack is the user program, not the JIT studied pattern |
| or anything else. The user program must ensure that if a stack is used |
| by pcre_exec(), (that is, it is assigned to the pattern currently run- |
| ning), that stack must not be used by any other threads (to avoid over- |
| writing the same memory area). The best practice for multithreaded pro- |
| grams is to allocate a stack for each thread, and return this stack |
| through the JIT callback function. |
| |
| (4) When should a JIT stack be freed? |
| |
| You can free a JIT stack at any time, as long as it will not be used by |
| pcre_exec() again. When you assign the stack to a pattern, only a |
| pointer is set. There is no reference counting or any other magic. You |
| can free the patterns and stacks in any order, anytime. Just do not |
| call pcre_exec() with a pattern pointing to an already freed stack, as |
| that will cause SEGFAULT. (Also, do not free a stack currently used by |
| pcre_exec() in another thread). You can also replace the stack for a |
| pattern at any time. You can even free the previous stack before |
| assigning a replacement. |
| |
| (5) Should I allocate/free a stack every time before/after calling |
| pcre_exec()? |
| |
| No, because this is too costly in terms of resources. However, you |
| could implement some clever idea which release the stack if it is not |
| used in let's say two minutes. The JIT callback can help to achive this |
| without keeping a list of the currently JIT studied patterns. |
| |
| (6) OK, the stack is for long term memory allocation. But what happens |
| if a pattern causes stack overflow with a stack of 1M? Is that 1M kept |
| until the stack is freed? |
| |
| Especially on embedded sytems, it might be a good idea to release mem- |
| ory sometimes without freeing the stack. There is no API for this at |
| the moment. Probably a function call which returns with the currently |
| allocated memory for any stack and another which allows releasing mem- |
| ory (shrinking the stack) would be a good idea if someone needs this. |
| |
| (7) This is too much of a headache. Isn't there any better solution for |
| JIT stack handling? |
| |
| No, thanks to Windows. If POSIX threads were used everywhere, we could |
| throw out this complicated API. |
| |
| |
| EXAMPLE CODE |
| |
| This is a single-threaded example that specifies a JIT stack without |
| using a callback. |
| |
| int rc; |
| int ovector[30]; |
| pcre *re; |
| pcre_extra *extra; |
| pcre_jit_stack *jit_stack; |
| |
| re = pcre_compile(pattern, 0, &error, &erroffset, NULL); |
| /* Check for errors */ |
| extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error); |
| jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024); |
| /* Check for error (NULL) */ |
| pcre_assign_jit_stack(extra, NULL, jit_stack); |
| rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30); |
| /* Check results */ |
| pcre_free(re); |
| pcre_free_study(extra); |
| pcre_jit_stack_free(jit_stack); |
| |
| |
| SEE ALSO |
| |
| pcreapi(3) |
| |
| |
| AUTHOR |
| |
| Philip Hazel (FAQ by Zoltan Herczeg) |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| |
| REVISION |
| |
| Last updated: 26 November 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| |
| |
| PCREPARTIAL(3) PCREPARTIAL(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions |
| |
| |
| PARTIAL MATCHING IN PCRE |
| |
| In normal use of PCRE, if the subject string that is passed to |
| pcre_exec() or pcre_dfa_exec() matches as far as it goes, but is too |
| short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. |
| There are circumstances where it might be helpful to distinguish this |
| case from other cases in which there is no match. |
| |
| Consider, for example, an application where a human is required to type |
| in data for a field with specific formatting requirements. An example |
| might be a date in the form ddmmmyy, defined by this pattern: |
| |
| ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$ |
| |
| If the application sees the user's keystrokes one by one, and can check |
| that what has been typed so far is potentially valid, it is able to |
| raise an error as soon as a mistake is made, by beeping and not |
| reflecting the character that has been typed, for example. This immedi- |
| ate feedback is likely to be a better user interface than a check that |
| is delayed until the entire string has been entered. Partial matching |
| can also be useful when the subject string is very long and is not all |
| available at once. |
| |
| PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and |
| PCRE_PARTIAL_HARD options, which can be set when calling pcre_exec() or |
| pcre_dfa_exec(). For backwards compatibility, PCRE_PARTIAL is a synonym |
| for PCRE_PARTIAL_SOFT. The essential difference between the two options |
| is whether or not a partial match is preferred to an alternative com- |
| plete match, though the details differ between the two matching func- |
| tions. If both options are set, PCRE_PARTIAL_HARD takes precedence. |
| |
| Setting a partial matching option for pcre_exec() disables the use of |
| any just-in-time code that was set up by calling pcre_study() with the |
| PCRE_STUDY_JIT_COMPILE option. It also disables two of PCRE's standard |
| optimizations. PCRE remembers the last literal byte in a pattern, and |
| abandons matching immediately if such a byte is not present in the sub- |
| ject string. This optimization cannot be used for a subject string that |
| might match only partially. If the pattern was studied, PCRE knows the |
| minimum length of a matching string, and does not bother to run the |
| matching function on shorter strings. This optimization is also dis- |
| abled for partial matching. |
| |
| |
| PARTIAL MATCHING USING pcre_exec() |
| |
| A partial match occurs during a call to pcre_exec() when the end of the |
| subject string is reached successfully, but matching cannot continue |
| because more characters are needed. However, at least one character in |
| the subject must have been inspected. This character need not form part |
| of the final matched string; lookbehind assertions and the \K escape |
| sequence provide ways of inspecting characters before the start of a |
| matched substring. The requirement for inspecting at least one charac- |
| ter exists because an empty string can always be matched; without such |
| a restriction there would always be a partial match of an empty string |
| at the end of the subject. |
| |
| If there are at least two slots in the offsets vector when pcre_exec() |
| returns with a partial match, the first slot is set to the offset of |
| the earliest character that was inspected when the partial match was |
| found. For convenience, the second offset points to the end of the sub- |
| ject so that a substring can easily be identified. |
| |
| For the majority of patterns, the first offset identifies the start of |
| the partially matched string. However, for patterns that contain look- |
| behind assertions, or \K, or begin with \b or \B, earlier characters |
| have been inspected while carrying out the match. For example: |
| |
| /(?<=abc)123/ |
| |
| This pattern matches "123", but only if it is preceded by "abc". If the |
| subject string is "xyzabc12", the offsets after a partial match are for |
| the substring "abc12", because all these characters are needed if |
| another match is tried with extra characters added to the subject. |
| |
| What happens when a partial match is identified depends on which of the |
| two partial matching options are set. |
| |
| PCRE_PARTIAL_SOFT with pcre_exec() |
| |
| If PCRE_PARTIAL_SOFT is set when pcre_exec() identifies a partial |
| match, the partial match is remembered, but matching continues as nor- |
| mal, and other alternatives in the pattern are tried. If no complete |
| match can be found, pcre_exec() returns PCRE_ERROR_PARTIAL instead of |
| PCRE_ERROR_NOMATCH. |
| |
| This option is "soft" because it prefers a complete match over a par- |
| tial match. All the various matching items in a pattern behave as if |
| the subject string is potentially complete. For example, \z, \Z, and $ |
| match at the end of the subject, as normal, and for \b and \B the end |
| of the subject is treated as a non-alphanumeric. |
| |
| If there is more than one partial match, the first one that was found |
| provides the data that is returned. Consider this pattern: |
| |
| /123\w+X|dogY/ |
| |
| If this is matched against the subject string "abc123dog", both alter- |
| natives fail to match, but the end of the subject is reached during |
| matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 |
| and 9, identifying "123dog" as the first partial match that was found. |
| (In this example, there are two partial matches, because "dog" on its |
| own partially matches the second alternative.) |
| |
| PCRE_PARTIAL_HARD with pcre_exec() |
| |
| If PCRE_PARTIAL_HARD is set for pcre_exec(), it returns PCRE_ERROR_PAR- |
| TIAL as soon as a partial match is found, without continuing to search |
| for possible complete matches. This option is "hard" because it prefers |
| an earlier partial match over a later complete match. For this reason, |
| the assumption is made that the end of the supplied subject string may |
| not be the true end of the available data, and so, if \z, \Z, \b, \B, |
| or $ are encountered at the end of the subject, the result is |
| PCRE_ERROR_PARTIAL. |
| |
| Setting PCRE_PARTIAL_HARD also affects the way pcre_exec() checks UTF-8 |
| subject strings for validity. Normally, an invalid UTF-8 sequence |
| causes the error PCRE_ERROR_BADUTF8. However, in the special case of a |
| truncated UTF-8 character at the end of the subject, PCRE_ERROR_SHORT- |
| UTF8 is returned when PCRE_PARTIAL_HARD is set. |
| |
| Comparing hard and soft partial matching |
| |
| The difference between the two partial matching options can be illus- |
| trated by a pattern such as: |
| |
| /dog(sbody)?/ |
| |
| This matches either "dog" or "dogsbody", greedily (that is, it prefers |
| the longer string if possible). If it is matched against the string |
| "dog" with PCRE_PARTIAL_SOFT, it yields a complete match for "dog". |
| However, if PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. |
| On the other hand, if the pattern is made ungreedy the result is dif- |
| ferent: |
| |
| /dog(sbody)??/ |
| |
| In this case the result is always a complete match because pcre_exec() |
| finds that first, and it never continues after finding a match. It |
| might be easier to follow this explanation by thinking of the two pat- |
| terns like this: |
| |
| /dog(sbody)?/ is the same as /dogsbody|dog/ |
| /dog(sbody)??/ is the same as /dog|dogsbody/ |
| |
| The second pattern will never match "dogsbody" when pcre_exec() is |
| used, because it will always find the shorter match first. |
| |
| |
| PARTIAL MATCHING USING pcre_dfa_exec() |
| |
| The pcre_dfa_exec() function moves along the subject string character |
| by character, without backtracking, searching for all possible matches |
| simultaneously. If the end of the subject is reached before the end of |
| the pattern, there is the possibility of a partial match, again pro- |
| vided that at least one character has been inspected. |
| |
| When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if |
| there have been no complete matches. Otherwise, the complete matches |
| are returned. However, if PCRE_PARTIAL_HARD is set, a partial match |
| takes precedence over any complete matches. The portion of the string |
| that was inspected when the longest partial match was found is set as |
| the first matching string, provided there are at least two slots in the |
| offsets vector. |
| |
| Because pcre_dfa_exec() always searches for all possible matches, and |
| there is no difference between greedy and ungreedy repetition, its be- |
| haviour is different from pcre_exec when PCRE_PARTIAL_HARD is set. Con- |
| sider the string "dog" matched against the ungreedy pattern shown |
| above: |
| |
| /dog(sbody)??/ |
| |
| Whereas pcre_exec() stops as soon as it finds the complete match for |
| "dog", pcre_dfa_exec() also finds the partial match for "dogsbody", and |
| so returns that when PCRE_PARTIAL_HARD is set. |
| |
| |
| PARTIAL MATCHING AND WORD BOUNDARIES |
| |
| If a pattern ends with one of sequences \b or \B, which test for word |
| boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter- |
| intuitive results. Consider this pattern: |
| |
| /\bcat\b/ |
| |
| This matches "cat", provided there is a word boundary at either end. If |
| the subject string is "the cat", the comparison of the final "t" with a |
| following character cannot take place, so a partial match is found. |
| However, pcre_exec() carries on with normal matching, which matches \b |
| at the end of the subject when the last character is a letter, thus |
| finding a complete match. The result, therefore, is not PCRE_ERROR_PAR- |
| TIAL. The same thing happens with pcre_dfa_exec(), because it also |
| finds the complete match. |
| |
| Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, |
| because then the partial match takes precedence. |
| |
| |
| FORMERLY RESTRICTED PATTERNS |
| |
| For releases of PCRE prior to 8.00, because of the way certain internal |
| optimizations were implemented in the pcre_exec() function, the |
| PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be |
| used with all patterns. From release 8.00 onwards, the restrictions no |
| longer apply, and partial matching with pcre_exec() can be requested |
| for any pattern. |
| |
| Items that were formerly restricted were repeated single characters and |
| repeated metasequences. If PCRE_PARTIAL was set for a pattern that did |
| not conform to the restrictions, pcre_exec() returned the error code |
| PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The |
| PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to find out if a compiled |
| pattern can be used for partial matching now always returns 1. |
| |
| |
| EXAMPLE OF PARTIAL MATCHING USING PCRETEST |
| |
| If the escape sequence \P is present in a pcretest data line, the |
| PCRE_PARTIAL_SOFT option is used for the match. Here is a run of |
| pcretest that uses the date example quoted above: |
| |
| re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ |
| data> 25jun04\P |
| 0: 25jun04 |
| 1: jun |
| data> 25dec3\P |
| Partial match: 23dec3 |
| data> 3ju\P |
| Partial match: 3ju |
| data> 3juj\P |
| No match |
| data> j\P |
| No match |
| |
| The first data string is matched completely, so pcretest shows the |
| matched substrings. The remaining four strings do not match the com- |
| plete pattern, but the first two are partial matches. Similar output is |
| obtained when pcre_dfa_exec() is used. |
| |
| If the escape sequence \P is present more than once in a pcretest data |
| line, the PCRE_PARTIAL_HARD option is set for the match. |
| |
| |
| MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() |
| |
| When a partial match has been found using pcre_dfa_exec(), it is possi- |
| ble to continue the match by providing additional subject data and |
| calling pcre_dfa_exec() again with the same compiled regular expres- |
| sion, this time setting the PCRE_DFA_RESTART option. You must pass the |
| same working space as before, because this is where details of the pre- |
| vious partial match are stored. Here is an example using pcretest, |
| using the \R escape sequence to set the PCRE_DFA_RESTART option (\D |
| specifies the use of pcre_dfa_exec()): |
| |
| re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ |
| data> 23ja\P\D |
| Partial match: 23ja |
| data> n05\R\D |
| 0: n05 |
| |
| The first call has "23ja" as the subject, and requests partial match- |
| ing; the second call has "n05" as the subject for the continued |
| (restarted) match. Notice that when the match is complete, only the |
| last part is shown; PCRE does not retain the previously partially- |
| matched string. It is up to the calling program to do that if it needs |
| to. |
| |
| You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with |
| PCRE_DFA_RESTART to continue partial matching over multiple segments. |
| This facility can be used to pass very long subject strings to |
| pcre_dfa_exec(). |
| |
| |
| MULTI-SEGMENT MATCHING WITH pcre_exec() |
| |
| From release 8.00, pcre_exec() can also be used to do multi-segment |
| matching. Unlike pcre_dfa_exec(), it is not possible to restart the |
| previous match with a new segment of data. Instead, new data must be |
| added to the previous subject string, and the entire match re-run, |
| starting from the point where the partial match occurred. Earlier data |
| can be discarded. It is best to use PCRE_PARTIAL_HARD in this situa- |
| tion, because it does not treat the end of a segment as the end of the |
| subject when matching \z, \Z, \b, \B, and $. Consider an unanchored |
| pattern that matches dates: |
| |
| re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/ |
| data> The date is 23ja\P\P |
| Partial match: 23ja |
| |
| At this stage, an application could discard the text preceding "23ja", |
| add on text from the next segment, and call pcre_exec() again. Unlike |
| pcre_dfa_exec(), the entire matching string must always be available, |
| and the complete matching process occurs for each call, so more memory |
| and more processing time is needed. |
| |
| Note: If the pattern contains lookbehind assertions, or \K, or starts |
| with \b or \B, the string that is returned for a partial match will |
| include characters that precede the partially matched string itself, |
| because these must be retained when adding on more characters for a |
| subsequent matching attempt. |
| |
| |
| ISSUES WITH MULTI-SEGMENT MATCHING |
| |
| Certain types of pattern may give problems with multi-segment matching, |
| whichever matching function is used. |
| |
| 1. If the pattern contains a test for the beginning of a line, you need |
| to pass the PCRE_NOTBOL option when the subject string for any call |
| does start at the beginning of a line. There is also a PCRE_NOTEOL |
| option, but in practice when doing multi-segment matching you should be |
| using PCRE_PARTIAL_HARD, which includes the effect of PCRE_NOTEOL. |
| |
| 2. Lookbehind assertions at the start of a pattern are catered for in |
| the offsets that are returned for a partial match. However, in theory, |
| a lookbehind assertion later in the pattern could require even earlier |
| characters to be inspected, and it might not have been reached when a |
| partial match occurs. This is probably an extremely unlikely case; you |
| could guard against it to a certain extent by always including extra |
| characters at the start. |
| |
| 3. Matching a subject string that is split into multiple segments may |
| not always produce exactly the same result as matching over one single |
| long string, especially when PCRE_PARTIAL_SOFT is used. The section |
| "Partial Matching and Word Boundaries" above describes an issue that |
| arises if the pattern ends with \b or \B. Another kind of difference |
| may occur when there are multiple matching possibilities, because (for |
| PCRE_PARTIAL_SOFT) a partial match result is given only when there are |
| no completed matches. This means that as soon as the shortest match has |
| been found, continuation to a new subject segment is no longer possi- |
| ble. Consider again this pcretest example: |
| |
| re> /dog(sbody)?/ |
| data> dogsb\P |
| 0: dog |
| data> do\P\D |
| Partial match: do |
| data> gsb\R\P\D |
| 0: g |
| data> dogsbody\D |
| 0: dogsbody |
| 1: dog |
| |
| The first data line passes the string "dogsb" to pcre_exec(), setting |
| the PCRE_PARTIAL_SOFT option. Although the string is a partial match |
| for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the |
| shorter string "dog" is a complete match. Similarly, when the subject |
| is presented to pcre_dfa_exec() in several parts ("do" and "gsb" being |
| the first two) the match stops when "dog" has been found, and it is not |
| possible to continue. On the other hand, if "dogsbody" is presented as |
| a single string, pcre_dfa_exec() finds both matches. |
| |
| Because of these problems, it is best to use PCRE_PARTIAL_HARD when |
| matching multi-segment data. The example above then behaves differ- |
| ently: |
| |
| re> /dog(sbody)?/ |
| data> dogsb\P\P |
| Partial match: dogsb |
| data> do\P\D |
| Partial match: do |
| data> gsb\R\P\P\D |
| Partial match: gsb |
| |
| 4. Patterns that contain alternatives at the top level which do not all |
| start with the same pattern item may not work as expected when |
| PCRE_DFA_RESTART is used with pcre_dfa_exec(). For example, consider |
| this pattern: |
| |
| 1234|3789 |
| |
| If the first part of the subject is "ABC123", a partial match of the |
| first alternative is found at offset 3. There is no partial match for |
| the second alternative, because such a match does not start at the same |
| point in the subject string. Attempting to continue with the string |
| "7890" does not yield a match because only those alternatives that |
| match at one point in the subject are remembered. The problem arises |
| because the start of the second alternative matches within the first |
| alternative. There is no problem with anchored patterns or patterns |
| such as: |
| |
| 1234|ABCD |
| |
| where no string can be a partial match for both alternatives. This is |
| not a problem if pcre_exec() is used, because the entire match has to |
| be rerun each time: |
| |
| re> /1234|3789/ |
| data> ABC123\P\P |
| Partial match: 123 |
| data> 1237890 |
| 0: 3789 |
| |
| Of course, instead of using PCRE_DFA_RESTART, the same technique of re- |
| running the entire match can also be used with pcre_dfa_exec(). Another |
| possibility is to work with two buffers. If a partial match at offset n |
| in the first buffer is followed by "no match" when PCRE_DFA_RESTART is |
| used on the second buffer, you can then try a new match starting at |
| offset n+1 in the first buffer. |
| |
| |
| AUTHOR |
| |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| |
| REVISION |
| |
| Last updated: 26 August 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| |
| |
| PCREPRECOMPILE(3) PCREPRECOMPILE(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions |
| |
| |
| SAVING AND RE-USING PRECOMPILED PCRE PATTERNS |
| |
| If you are running an application that uses a large number of regular |
| expression patterns, it may be useful to store them in a precompiled |
| form instead of having to compile them every time the application is |
| run. If you are not using any private character tables (see the |
| pcre_maketables() documentation), this is relatively straightforward. |
| If you are using private tables, it is a little bit more complicated. |
| However, if you are using the just-in-time optimization feature of |
| pcre_study(), it is not possible to save and reload the JIT data. |
| |
| If you save compiled patterns to a file, you can copy them to a differ- |
| ent host and run them there. This works even if the new host has the |
| opposite endianness to the one on which the patterns were compiled. |
| There may be a small performance penalty, but it should be insignifi- |
| cant. However, compiling regular expressions with one version of PCRE |
| for use with a different version is not guaranteed to work and may |
| cause crashes, and saving and restoring a compiled pattern loses any |
| JIT optimization data. |
| |
| |
| SAVING A COMPILED PATTERN |
| |
| The value returned by pcre_compile() points to a single block of memory |
| that holds the compiled pattern and associated data. You can find the |
| length of this block in bytes by calling pcre_fullinfo() with an argu- |
| ment of PCRE_INFO_SIZE. You can then save the data in any appropriate |
| manner. Here is sample code that compiles a pattern and writes it to a |
| file. It assumes that the variable fd refers to a file that is open for |
| output: |
| |
| int erroroffset, rc, size; |
| char *error; |
| pcre *re; |
| |
| re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL); |
| if (re == NULL) { ... handle errors ... } |
| rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size); |
| if (rc < 0) { ... handle errors ... } |
| rc = fwrite(re, 1, size, fd); |
| if (rc != size) { ... handle errors ... } |
| |
| In this example, the bytes that comprise the compiled pattern are |
| copied exactly. Note that this is binary data that may contain any of |
| the 256 possible byte values. On systems that make a distinction |
| between binary and non-binary data, be sure that the file is opened for |
| binary output. |
| |
| If you want to write more than one pattern to a file, you will have to |
| devise a way of separating them. For binary data, preceding each pat- |
| tern with its length is probably the most straightforward approach. |
| Another possibility is to write out the data in hexadecimal instead of |
| binary, one pattern to a line. |
| |
| Saving compiled patterns in a file is only one possible way of storing |
| them for later use. They could equally well be saved in a database, or |
| in the memory of some daemon process that passes them via sockets to |
| the processes that want them. |
| |
| If the pattern has been studied, it is also possible to save the normal |
| study data in a similar way to the compiled pattern itself. However, if |
| the PCRE_STUDY_JIT_COMPILE was used, the just-in-time data that is cre- |
| ated cannot be saved because it is too dependent on the current envi- |
| ronment. When studying generates additional information, pcre_study() |
| returns a pointer to a pcre_extra data block. Its format is defined in |
| the section on matching a pattern in the pcreapi documentation. The |
| study_data field points to the binary study data, and this is what you |
| must save (not the pcre_extra block itself). The length of the study |
| data can be obtained by calling pcre_fullinfo() with an argument of |
| PCRE_INFO_STUDYSIZE. Remember to check that pcre_study() did return a |
| non-NULL value before trying to save the study data. |
| |
| |
| RE-USING A PRECOMPILED PATTERN |
| |
| Re-using a precompiled pattern is straightforward. Having reloaded it |
| into main memory, you pass its pointer to pcre_exec() or |
| pcre_dfa_exec() in the usual way. This should work even on another |
| host, and even if that host has the opposite endianness to the one |
| where the pattern was compiled. |
| |
| However, if you passed a pointer to custom character tables when the |
| pattern was compiled (the tableptr argument of pcre_compile()), you |
| must now pass a similar pointer to pcre_exec() or pcre_dfa_exec(), |
| because the value saved with the compiled pattern will obviously be |
| nonsense. A field in a pcre_extra() block is used to pass this data, as |
| described in the section on matching a pattern in the pcreapi documen- |
| tation. |
| |
| If you did not provide custom character tables when the pattern was |
| compiled, the pointer in the compiled pattern is NULL, which causes |
| pcre_exec() to use PCRE's internal tables. Thus, you do not need to |
| take any special action at run time in this case. |
| |
| If you saved study data with the compiled pattern, you need to create |
| your own pcre_extra data block and set the study_data field to point to |
| the reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA |
| bit in the flags field to indicate that study data is present. Then |
| pass the pcre_extra block to pcre_exec() or pcre_dfa_exec() in the |
| usual way. If the pattern was studied for just-in-time optimization, |
| that data cannot be saved, and so is lost by a save/restore cycle. |
| |
| |
| COMPATIBILITY WITH DIFFERENT PCRE RELEASES |
| |
| In general, it is safest to recompile all saved patterns when you |
| update to a new PCRE release, though not all updates actually require |
| this. |
| |
| |
| AUTHOR |
| |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| |
| REVISION |
| |
| Last updated: 26 August 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| |
| |
| PCREPERFORM(3) PCREPERFORM(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions |
| |
| |
| PCRE PERFORMANCE |
| |
| Two aspects of performance are discussed below: memory usage and pro- |
| cessing time. The way you express your pattern as a regular expression |
| can affect both of them. |
| |
| |
| COMPILED PATTERN MEMORY USAGE |
| |
| Patterns are compiled by PCRE into a reasonably efficient byte code, so |
| that most simple patterns do not use much memory. However, there is one |
| case where the memory usage of a compiled pattern can be unexpectedly |
| large. If a parenthesized subpattern has a quantifier with a minimum |
| greater than 1 and/or a limited maximum, the whole subpattern is |
| repeated in the compiled code. For example, the pattern |
| |
| (abc|def){2,4} |
| |
| is compiled as if it were |
| |
| (abc|def)(abc|def)((abc|def)(abc|def)?)? |
| |
| (Technical aside: It is done this way so that backtrack points within |
| each of the repetitions can be independently maintained.) |
| |
| For regular expressions whose quantifiers use only small numbers, this |
| is not usually a problem. However, if the numbers are large, and par- |
| ticularly if such repetitions are nested, the memory usage can become |
| an embarrassment. For example, the very simple pattern |
| |
| ((ab){1,1000}c){1,3} |
| |
| uses 51K bytes when compiled. When PCRE is compiled with its default |
| internal pointer size of two bytes, the size limit on a compiled pat- |
| tern is 64K, and this is reached with the above pattern if the outer |
| repetition is increased from 3 to 4. PCRE can be compiled to use larger |
| internal pointers and thus handle larger compiled patterns, but it is |
| better to try to rewrite your pattern to use less memory if you can. |
| |
| One way of reducing the memory usage for such patterns is to make use |
| of PCRE's "subroutine" facility. Re-writing the above pattern as |
| |
| ((ab)(?2){0,999}c)(?1){0,2} |
| |
| reduces the memory requirements to 18K, and indeed it remains under 20K |
| even with the outer repetition increased to 100. However, this pattern |
| is not exactly equivalent, because the "subroutine" calls are treated |
| as atomic groups into which there can be no backtracking if there is a |
| subsequent matching failure. Therefore, PCRE cannot do this kind of |
| rewriting automatically. Furthermore, there is a noticeable loss of |
| speed when executing the modified pattern. Nevertheless, if the atomic |
| grouping is not a problem and the loss of speed is acceptable, this |
| kind of rewriting will allow you to process patterns that PCRE cannot |
| otherwise handle. |
| |
| |
| STACK USAGE AT RUN TIME |
| |
| When pcre_exec() is used for matching, certain kinds of pattern can |
| cause it to use large amounts of the process stack. In some environ- |
| ments the default process stack is quite small, and if it runs out the |
| result is often SIGSEGV. This issue is probably the most frequently |
| raised problem with PCRE. Rewriting your pattern can often help. The |
| pcrestack documentation discusses this issue in detail. |
| |
| |
| PROCESSING TIME |
| |
| Certain items in regular expression patterns are processed more effi- |
| ciently than others. It is more efficient to use a character class like |
| [aeiou] than a set of single-character alternatives such as |
| (a|e|i|o|u). In general, the simplest construction that provides the |
| required behaviour is usually the most efficient. Jeffrey Friedl's book |
| contains a lot of useful general discussion about optimizing regular |
| expressions for efficient performance. This document contains a few |
| observations about PCRE. |
| |
| Using Unicode character properties (the \p, \P, and \X escapes) is |
| slow, because PCRE has to scan a structure that contains data for over |
| fifteen thousand characters whenever it needs a character's property. |
| If you can find an alternative pattern that does not use character |
| properties, it will probably be faster. |
| |
| By default, the escape sequences \b, \d, \s, and \w, and the POSIX |
| character classes such as [:alpha:] do not use Unicode properties, |
| partly for backwards compatibility, and partly for performance reasons. |
| However, you can set PCRE_UCP if you want Unicode character properties |
| to be used. This can double the matching time for items such as \d, |
| when matched with pcre_exec(); the performance loss is less with |
| pcre_dfa_exec(), and in both cases there is not much difference for \b. |
| |
| When a pattern begins with .* not in parentheses, or in parentheses |
| that are not the subject of a backreference, and the PCRE_DOTALL option |
| is set, the pattern is implicitly anchored by PCRE, since it can match |
| only at the start of a subject string. However, if PCRE_DOTALL is not |
| set, PCRE cannot make this optimization, because the . metacharacter |
| does not then match a newline, and if the subject string contains new- |
| lines, the pattern may match from the character immediately following |
| one of them instead of from the very start. For example, the pattern |
| |
| .*second |
| |
| matches the subject "first\nand second" (where \n stands for a newline |
| character), with the match starting at the seventh character. In order |
| to do this, PCRE has to retry the match starting after every newline in |
| the subject. |
| |
| If you are using such a pattern with subject strings that do not con- |
| tain newlines, the best performance is obtained by setting PCRE_DOTALL, |
| or starting the pattern with ^.* or ^.*? to indicate explicit anchor- |
| ing. That saves PCRE from having to scan along the subject looking for |
| a newline to restart at. |
| |
| Beware of patterns that contain nested indefinite repeats. These can |
| take a long time to run when applied to a string that does not match. |
| Consider the pattern fragment |
| |
| ^(a+)* |
| |
| This can match "aaaa" in 16 different ways, and this number increases |
| very rapidly as the string gets longer. (The * repeat can match 0, 1, |
| 2, 3, or 4 times, and for each of those cases other than 0 or 4, the + |
| repeats can match different numbers of times.) When the remainder of |
| the pattern is such that the entire match is going to fail, PCRE has in |
| principle to try every possible variation, and this can take an |
| extremely long time, even for relatively short strings. |
| |
| An optimization catches some of the more simple cases such as |
| |
| (a+)*b |
| |
| where a literal character follows. Before embarking on the standard |
| matching procedure, PCRE checks that there is a "b" later in the sub- |
| ject string, and if there is not, it fails the match immediately. How- |
| ever, when there is no following literal this optimization cannot be |
| used. You can see the difference by comparing the behaviour of |
| |
| (a+)*\d |
| |
| with the pattern above. The former gives a failure almost instantly |
| when applied to a whole line of "a" characters, whereas the latter |
| takes an appreciable time with strings longer than about 20 characters. |
| |
| In many cases, the solution to this kind of performance issue is to use |
| an atomic group or a possessive quantifier. |
| |
| |
| AUTHOR |
| |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| |
| REVISION |
| |
| Last updated: 16 May 2010 |
| Copyright (c) 1997-2010 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| |
| |
| PCREPOSIX(3) PCREPOSIX(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions. |
| |
| |
| SYNOPSIS OF POSIX API |
| |
| #include <pcreposix.h> |
| |
| int regcomp(regex_t *preg, const char *pattern, |
| int cflags); |
| |
| int regexec(regex_t *preg, const char *string, |
| size_t nmatch, regmatch_t pmatch[], int eflags); |
| |
| size_t regerror(int errcode, const regex_t *preg, |
| char *errbuf, size_t errbuf_size); |
| |
| void regfree(regex_t *preg); |
| |
| |
| DESCRIPTION |
| |
| This set of functions provides a POSIX-style API to the PCRE regular |
| expression package. See the pcreapi documentation for a description of |
| PCRE's native API, which contains much additional functionality. |
| |
| The functions described here are just wrapper functions that ultimately |
| call the PCRE native API. Their prototypes are defined in the |
| pcreposix.h header file, and on Unix systems the library itself is |
| called pcreposix.a, so can be accessed by adding -lpcreposix to the |
| command for linking an application that uses them. Because the POSIX |
| functions call the native ones, it is also necessary to add -lpcre. |
| |
| I have implemented only those POSIX option bits that can be reasonably |
| mapped to PCRE native options. In addition, the option REG_EXTENDED is |
| defined with the value zero. This has no effect, but since programs |
| that are written to the POSIX interface often use it, this makes it |
| easier to slot in PCRE as a replacement library. Other POSIX options |
| are not even defined. |
| |
| There are also some other options that are not defined by POSIX. These |
| have been added at the request of users who want to make use of certain |
| PCRE-specific features via the POSIX calling interface. |
| |
| When PCRE is called via these functions, it is only the API that is |
| POSIX-like in style. The syntax and semantics of the regular expres- |
| sions themselves are still those of Perl, subject to the setting of |
| various PCRE options, as described below. "POSIX-like in style" means |
| that the API approximates to the POSIX definition; it is not fully |
| POSIX-compatible, and in multi-byte encoding domains it is probably |
| even less compatible. |
| |
| The header for these functions is supplied as pcreposix.h to avoid any |
| potential clash with other POSIX libraries. It can, of course, be |
| renamed or aliased as regex.h, which is the "correct" name. It provides |
| two structure types, regex_t for compiled internal forms, and reg- |
| match_t for returning captured substrings. It also defines some con- |
| stants whose names start with "REG_"; these are used for setting |
| options and identifying error codes. |
| |
| |
| COMPILING A PATTERN |
| |
| The function regcomp() is called to compile a pattern into an internal |
| form. The pattern is a C string terminated by a binary zero, and is |
| passed in the argument pattern. The preg argument is a pointer to a |
| regex_t structure that is used as a base for storing information about |
| the compiled regular expression. |
| |
| The argument cflags is either zero, or contains one or more of the bits |
| defined by the following macros: |
| |
| REG_DOTALL |
| |
| The PCRE_DOTALL option is set when the regular expression is passed for |
| compilation to the native function. Note that REG_DOTALL is not part of |
| the POSIX standard. |
| |
| REG_ICASE |
| |
| The PCRE_CASELESS option is set when the regular expression is passed |
| for compilation to the native function. |
| |
| REG_NEWLINE |
| |
| The PCRE_MULTILINE option is set when the regular expression is passed |
| for compilation to the native function. Note that this does not mimic |
| the defined POSIX behaviour for REG_NEWLINE (see the following sec- |
| tion). |
| |
| REG_NOSUB |
| |
| The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is |
| passed for compilation to the native function. In addition, when a pat- |
| tern that is compiled with this flag is passed to regexec() for match- |
| ing, the nmatch and pmatch arguments are ignored, and no captured |
| strings are returned. |
| |
| REG_UCP |
| |
| The PCRE_UCP option is set when the regular expression is passed for |
| compilation to the native function. This causes PCRE to use Unicode |
| properties when matchine \d, \w, etc., instead of just recognizing |
| ASCII values. Note that REG_UTF8 is not part of the POSIX standard. |
| |
| REG_UNGREEDY |
| |
| The PCRE_UNGREEDY option is set when the regular expression is passed |
| for compilation to the native function. Note that REG_UNGREEDY is not |
| part of the POSIX standard. |
| |
| REG_UTF8 |
| |
| The PCRE_UTF8 option is set when the regular expression is passed for |
| compilation to the native function. This causes the pattern itself and |
| all data strings used for matching it to be treated as UTF-8 strings. |
| Note that REG_UTF8 is not part of the POSIX standard. |
| |
| In the absence of these flags, no options are passed to the native |
| function. This means the the regex is compiled with PCRE default |
| semantics. In particular, the way it handles newline characters in the |
| subject string is the Perl way, not the POSIX way. Note that setting |
| PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE. |
| It does not affect the way newlines are matched by . (they are not) or |
| by a negative class such as [^a] (they are). |
| |
| The yield of regcomp() is zero on success, and non-zero otherwise. The |
| preg structure is filled in on success, and one member of the structure |
| is public: re_nsub contains the number of capturing subpatterns in the |
| regular expression. Various error codes are defined in the header file. |
| |
| NOTE: If the yield of regcomp() is non-zero, you must not attempt to |
| use the contents of the preg structure. If, for example, you pass it to |
| regexec(), the result is undefined and your program is likely to crash. |
| |
| |
| MATCHING NEWLINE CHARACTERS |
| |
| This area is not simple, because POSIX and Perl take different views of |
| things. It is not possible to get PCRE to obey POSIX semantics, but |
| then PCRE was never intended to be a POSIX engine. The following table |
| lists the different possibilities for matching newline characters in |
| PCRE: |
| |
| Default Change with |
| |
| . matches newline no PCRE_DOTALL |
| newline matches [^a] yes not changeable |
| $ matches \n at end yes PCRE_DOLLARENDONLY |
| $ matches \n in middle no PCRE_MULTILINE |
| ^ matches \n in middle no PCRE_MULTILINE |
| |
| This is the equivalent table for POSIX: |
| |
| Default Change with |
| |
| . matches newline yes REG_NEWLINE |
| newline matches [^a] yes REG_NEWLINE |
| $ matches \n at end no REG_NEWLINE |
| $ matches \n in middle no REG_NEWLINE |
| ^ matches \n in middle no REG_NEWLINE |
| |
| PCRE's behaviour is the same as Perl's, except that there is no equiva- |
| lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is |
| no way to stop newline from matching [^a]. |
| |
| The default POSIX newline handling can be obtained by setting |
| PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE |
| behave exactly as for the REG_NEWLINE action. |
| |
| |
| MATCHING A PATTERN |
| |
| The function regexec() is called to match a compiled pattern preg |
| against a given string, which is by default terminated by a zero byte |
| (but see REG_STARTEND below), subject to the options in eflags. These |
| can be: |
| |
| REG_NOTBOL |
| |
| The PCRE_NOTBOL option is set when calling the underlying PCRE matching |
| function. |
| |
| REG_NOTEMPTY |
| |
| The PCRE_NOTEMPTY option is set when calling the underlying PCRE match- |
| ing function. Note that REG_NOTEMPTY is not part of the POSIX standard. |
| However, setting this option can give more POSIX-like behaviour in some |
| situations. |
| |
| REG_NOTEOL |
| |
| The PCRE_NOTEOL option is set when calling the underlying PCRE matching |
| function. |
| |
| REG_STARTEND |
| |
| The string is considered to start at string + pmatch[0].rm_so and to |
| have a terminating NUL located at string + pmatch[0].rm_eo (there need |
| not actually be a NUL at that location), regardless of the value of |
| nmatch. This is a BSD extension, compatible with but not specified by |
| IEEE Standard 1003.2 (POSIX.2), and should be used with caution in |
| software intended to be portable to other systems. Note that a non-zero |
| rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location |
| of the string, not how it is matched. |
| |
| If the pattern was compiled with the REG_NOSUB flag, no data about any |
| matched strings is returned. The nmatch and pmatch arguments of |
| regexec() are ignored. |
| |
| If the value of nmatch is zero, or if the value pmatch is NULL, no data |
| about any matched strings is returned. |
| |
| Otherwise,the portion of the string that was matched, and also any cap- |
| tured substrings, are returned via the pmatch argument, which points to |
| an array of nmatch structures of type regmatch_t, containing the mem- |
| bers rm_so and rm_eo. These contain the offset to the first character |
| of each substring and the offset to the first character after the end |
| of each substring, respectively. The 0th element of the vector relates |
| to the entire portion of string that was matched; subsequent elements |
| relate to the capturing subpatterns of the regular expression. Unused |
| entries in the array have both structure members set to -1. |
| |
| A successful match yields a zero return; various error codes are |
| defined in the header file, of which REG_NOMATCH is the "expected" |
| failure code. |
| |
| |
| ERROR MESSAGES |
| |
| The regerror() function maps a non-zero errorcode from either regcomp() |
| or regexec() to a printable message. If preg is not NULL, the error |
| should have arisen from the use of that structure. A message terminated |
| by a binary zero is placed in errbuf. The length of the message, |
| including the zero, is limited to errbuf_size. The yield of the func- |
| tion is the size of buffer needed to hold the whole message. |
| |
| |
| MEMORY USAGE |
| |
| Compiling a regular expression causes memory to be allocated and asso- |
| ciated with the preg structure. The function regfree() frees all such |
| memory, after which preg may no longer be used as a compiled expres- |
| sion. |
| |
| |
| AUTHOR |
| |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| |
| REVISION |
| |
| Last updated: 16 May 2010 |
| Copyright (c) 1997-2010 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| |
| |
| PCRECPP(3) PCRECPP(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions. |
| |
| |
| SYNOPSIS OF C++ WRAPPER |
| |
| #include <pcrecpp.h> |
| |
| |
| DESCRIPTION |
| |
| The C++ wrapper for PCRE was provided by Google Inc. Some additional |
| functionality was added by Giuseppe Maxia. This brief man page was con- |
| structed from the notes in the pcrecpp.h file, which should be con- |
| sulted for further details. |
| |
| |
| MATCHING INTERFACE |
| |
| The "FullMatch" operation checks that supplied text matches a supplied |
| pattern exactly. If pointer arguments are supplied, it copies matched |
| sub-strings that match sub-patterns into them. |
| |
| Example: successful match |
| pcrecpp::RE re("h.*o"); |
| re.FullMatch("hello"); |
| |
| Example: unsuccessful match (requires full match): |
| pcrecpp::RE re("e"); |
| !re.FullMatch("hello"); |
| |
| Example: creating a temporary RE object: |
| pcrecpp::RE("h.*o").FullMatch("hello"); |
| |
| You can pass in a "const char*" or a "string" for "text". The examples |
| below tend to use a const char*. You can, as in the different examples |
| above, store the RE object explicitly in a variable or use a temporary |
| RE object. The examples below use one mode or the other arbitrarily. |
| Either could correctly be used for any of these examples. |
| |
| You must supply extra pointer arguments to extract matched subpieces. |
| |
| Example: extracts "ruby" into "s" and 1234 into "i" |
| int i; |
| string s; |
| pcrecpp::RE re("(\\w+):(\\d+)"); |
| re.FullMatch("ruby:1234", &s, &i); |
| |
| Example: does not try to extract any extra sub-patterns |
| re.FullMatch("ruby:1234", &s); |
| |
| Example: does not try to extract into NULL |
| re.FullMatch("ruby:1234", NULL, &i); |
| |
| Example: integer overflow causes failure |
| !re.FullMatch("ruby:1234567891234", NULL, &i); |
| |
| Example: fails because there aren't enough sub-patterns: |
| !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s); |
| |
| Example: fails because string cannot be stored in integer |
| !pcrecpp::RE("(.*)").FullMatch("ruby", &i); |
| |
| The provided pointer arguments can be pointers to any scalar numeric |
| type, or one of: |
| |
| string (matched piece is copied to string) |
| StringPiece (StringPiece is mutated to point to matched piece) |
| T (where "bool T::ParseFrom(const char*, int)" exists) |
| NULL (the corresponding matched sub-pattern is not copied) |
| |
| The function returns true iff all of the following conditions are sat- |
| isfied: |
| |
| a. "text" matches "pattern" exactly; |
| |
| b. The number of matched sub-patterns is >= number of supplied |
| pointers; |
| |
| c. The "i"th argument has a suitable type for holding the |
| string captured as the "i"th sub-pattern. If you pass in |
| void * NULL for the "i"th argument, or a non-void * NULL |
| of the correct type, or pass fewer arguments than the |
| number of sub-patterns, "i"th captured sub-pattern is |
| ignored. |
| |
| CAVEAT: An optional sub-pattern that does not exist in the matched |
| string is assigned the empty string. Therefore, the following will |
| return false (because the empty string is not a valid number): |
| |
| int number; |
| pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number); |
| |
| The matching interface supports at most 16 arguments per call. If you |
| need more, consider using the more general interface |
| pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch. |
| |
| NOTE: Do not use no_arg, which is used internally to mark the end of a |
| list of optional arguments, as a placeholder for missing arguments, as |
| this can lead to segfaults. |
| |
| |
| QUOTING METACHARACTERS |
| |
| You can use the "QuoteMeta" operation to insert backslashes before all |
| potentially meaningful characters in a string. The returned string, |
| used as a regular expression, will exactly match the original string. |
| |
| Example: |
| string quoted = RE::QuoteMeta(unquoted); |
| |
| Note that it's legal to escape a character even if it has no special |
| meaning in a regular expression -- so this function does that. (This |
| also makes it identical to the perl function of the same name; see |
| "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes |
| "1\.5\-2\.0\?". |
| |
| |
| PARTIAL MATCHES |
| |
| You can use the "PartialMatch" operation when you want the pattern to |
| match any substring of the text. |
| |
| Example: simple search for a string: |
| pcrecpp::RE("ell").PartialMatch("hello"); |
| |
| Example: find first number in a string: |
| int number; |
| pcrecpp::RE re("(\\d+)"); |
| re.PartialMatch("x*100 + 20", &number); |
| assert(number == 100); |
| |
| |
| UTF-8 AND THE MATCHING INTERFACE |
| |
| By default, pattern and text are plain text, one byte per character. |
| The UTF8 flag, passed to the constructor, causes both pattern and |
| string to be treated as UTF-8 text, still a byte stream but potentially |
| multiple bytes per character. In practice, the text is likelier to be |
| UTF-8 than the pattern, but the match returned may depend on the UTF8 |
| flag, so always use it when matching UTF8 text. For example, "." will |
| match one byte normally but with UTF8 set may match up to three bytes |
| of a multi-byte character. |
| |
| Example: |
| pcrecpp::RE_Options options; |
| options.set_utf8(); |
| pcrecpp::RE re(utf8_pattern, options); |
| re.FullMatch(utf8_string); |
| |
| Example: using the convenience function UTF8(): |
| pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8()); |
| re.FullMatch(utf8_string); |
| |
| NOTE: The UTF8 flag is ignored if pcre was not configured with the |
| --enable-utf8 flag. |
| |
| |
| PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE |
| |
| PCRE defines some modifiers to change the behavior of the regular |
| expression engine. The C++ wrapper defines an auxiliary class, |
| RE_Options, as a vehicle to pass such modifiers to a RE class. Cur- |
| rently, the following modifiers are supported: |
| |
| modifier description Perl corresponding |
| |
| PCRE_CASELESS case insensitive match /i |
| PCRE_MULTILINE multiple lines match /m |
| PCRE_DOTALL dot matches newlines /s |
| PCRE_DOLLAR_ENDONLY $ matches only at end N/A |
| PCRE_EXTRA strict escape parsing N/A |
| PCRE_EXTENDED ignore whitespaces /x |
| PCRE_UTF8 handles UTF8 chars built-in |
| PCRE_UNGREEDY reverses * and *? N/A |
| PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*) |
| |
| (*) Both Perl and PCRE allow non capturing parentheses by means of the |
| "?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap- |
| ture, while (ab|cd) does. |
| |
| For a full account on how each modifier works, please check the PCRE |
| API reference page. |
| |
| For each modifier, there are two member functions whose name is made |
| out of the modifier in lowercase, without the "PCRE_" prefix. For |
| instance, PCRE_CASELESS is handled by |
| |
| bool caseless() |
| |
| which returns true if the modifier is set, and |
| |
| RE_Options & set_caseless(bool) |
| |
| which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can |
| be accessed through the set_match_limit() and match_limit() member |
| functions. Setting match_limit to a non-zero value will limit the exe- |
| cution of pcre to keep it from doing bad things like blowing the stack |
| or taking an eternity to return a result. A value of 5000 is good |
| enough to stop stack blowup in a 2MB thread stack. Setting match_limit |
| to zero disables match limiting. Alternatively, you can call |
| match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to |
| limit how much PCRE recurses. match_limit() limits the number of |
| matches PCRE does; match_limit_recursion() limits the depth of internal |
| recursion, and therefore the amount of stack that is used. |
| |
| Normally, to pass one or more modifiers to a RE class, you declare a |
| RE_Options object, set the appropriate options, and pass this object to |
| a RE constructor. Example: |
| |
| RE_Options opt; |
| opt.set_caseless(true); |
| if (RE("HELLO", opt).PartialMatch("hello world")) ... |
| |
| RE_options has two constructors. The default constructor takes no argu- |
| ments and creates a set of flags that are off by default. The optional |
| parameter option_flags is to facilitate transfer of legacy code from C |
| programs. This lets you do |
| |
| RE(pattern, |
| RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str); |
| |
| However, new code is better off doing |
| |
| RE(pattern, |
| RE_Options().set_caseless(true).set_multiline(true)) |
| .PartialMatch(str); |
| |
| If you are going to pass one of the most used modifiers, there are some |
| convenience functions that return a RE_Options class with the appropri- |
| ate modifier already set: CASELESS(), UTF8(), MULTILINE(), DOTALL(), |
| and EXTENDED(). |
| |
| If you need to set several options at once, and you don't want to go |
| through the pains of declaring a RE_Options object and setting several |
| options, there is a parallel method that give you such ability on the |
| fly. You can concatenate several set_xxxxx() member functions, since |
| each of them returns a reference to its class object. For example, to |
| pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one |
| statement, you may write: |
| |
| RE(" ^ xyz \\s+ .* blah$", |
| RE_Options() |
| .set_caseless(true) |
| .set_extended(true) |
| .set_multiline(true)).PartialMatch(sometext); |
| |
| |
| SCANNING TEXT INCREMENTALLY |
| |
| The "Consume" operation may be useful if you want to repeatedly match |
| regular expressions at the front of a string and skip over them as they |
| match. This requires use of the "StringPiece" type, which represents a |
| sub-range of a real string. Like RE, StringPiece is defined in the |
| pcrecpp namespace. |
| |
| Example: read lines of the form "var = value" from a string. |
| string contents = ...; // Fill string somehow |
| pcrecpp::StringPiece input(contents); // Wrap in a StringPiece |
| |
| string var; |
| int value; |
| pcrecpp::RE re("(\\w+) = (\\d+)\n"); |
| while (re.Consume(&input, &var, &value)) { |
| ...; |
| } |
| |
| Each successful call to "Consume" will set "var/value", and also |
| advance "input" so it points past the matched text. |
| |
| The "FindAndConsume" operation is similar to "Consume" but does not |
| anchor your match at the beginning of the string. For example, you |
| could extract all words from a string by repeatedly calling |
| |
| pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word) |
| |
| |
| PARSING HEX/OCTAL/C-RADIX NUMBERS |
| |
| By default, if you pass a pointer to a numeric value, the corresponding |
| text is interpreted as a base-10 number. You can instead wrap the |
| pointer with a call to one of the operators Hex(), Octal(), or CRadix() |
| to interpret the text in another base. The CRadix operator interprets |
| C-style "0" (base-8) and "0x" (base-16) prefixes, but defaults to |
| base-10. |
| |
| Example: |
| int a, b, c, d; |
| pcrecpp::RE re("(.*) (.*) (.*) (.*)"); |
| re.FullMatch("100 40 0100 0x40", |
| pcrecpp::Octal(&a), pcrecpp::Hex(&b), |
| pcrecpp::CRadix(&c), pcrecpp::CRadix(&d)); |
| |
| will leave 64 in a, b, c, and d. |
| |
| |
| REPLACING PARTS OF STRINGS |
| |
| You can replace the first match of "pattern" in "str" with "rewrite". |
| Within "rewrite", backslash-escaped digits (\1 to \9) can be used to |
| insert text matching corresponding parenthesized group from the pat- |
| tern. \0 in "rewrite" refers to the entire matching text. For example: |
| |
| string s = "yabba dabba doo"; |
| pcrecpp::RE("b+").Replace("d", &s); |
| |
| will leave "s" containing "yada dabba doo". The result is true if the |
| pattern matches and a replacement occurs, false otherwise. |
| |
| GlobalReplace is like Replace except that it replaces all occurrences |
| of the pattern in the string with the rewrite. Replacements are not |
| subject to re-matching. For example: |
| |
| string s = "yabba dabba doo"; |
| pcrecpp::RE("b+").GlobalReplace("d", &s); |
| |
| will leave "s" containing "yada dada doo". It returns the number of |
| replacements made. |
| |
| Extract is like Replace, except that if the pattern matches, "rewrite" |
| is copied into "out" (an additional argument) with substitutions. The |
| non-matching portions of "text" are ignored. Returns true iff a match |
| occurred and the extraction happened successfully; if no match occurs, |
| the string is left unaffected. |
| |
| |
| AUTHOR |
| |
| The C++ wrapper was contributed by Google Inc. |
| Copyright (c) 2007 Google Inc. |
| |
| |
| REVISION |
| |
| Last updated: 17 March 2009 |
| Minor typo fixed: 25 July 2011 |
| ------------------------------------------------------------------------------ |
| |
| |
| PCRESAMPLE(3) PCRESAMPLE(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions |
| |
| |
| PCRE SAMPLE PROGRAM |
| |
| A simple, complete demonstration program, to get you started with using |
| PCRE, is supplied in the file pcredemo.c in the PCRE distribution. A |
| listing of this program is given in the pcredemo documentation. If you |
| do not have a copy of the PCRE distribution, you can save this listing |
| to re-create pcredemo.c. |
| |
| The program compiles the regular expression that is its first argument, |
| and matches it against the subject string in its second argument. No |
| PCRE options are set, and default character tables are used. If match- |
| ing succeeds, the program outputs the portion of the subject that |
| matched, together with the contents of any captured substrings. |
| |
| If the -g option is given on the command line, the program then goes on |
| to check for further matches of the same regular expression in the same |
| subject string. The logic is a little bit tricky because of the possi- |
| bility of matching an empty string. Comments in the code explain what |
| is going on. |
| |
| If PCRE is installed in the standard include and library directories |
| for your operating system, you should be able to compile the demonstra- |
| tion program using this command: |
| |
| gcc -o pcredemo pcredemo.c -lpcre |
| |
| If PCRE is installed elsewhere, you may need to add additional options |
| to the command line. For example, on a Unix-like system that has PCRE |
| installed in /usr/local, you can compile the demonstration program |
| using a command like this: |
| |
| gcc -o pcredemo -I/usr/local/include pcredemo.c \ |
| -L/usr/local/lib -lpcre |
| |
| In a Windows environment, if you want to statically link the program |
| against a non-dll pcre.a file, you must uncomment the line that defines |
| PCRE_STATIC before including pcre.h, because otherwise the pcre_mal- |
| loc() and pcre_free() exported functions will be declared |
| __declspec(dllimport), with unwanted results. |
| |
| Once you have compiled and linked the demonstration program, you can |
| run simple tests like this: |
| |
| ./pcredemo 'cat|dog' 'the cat sat on the mat' |
| ./pcredemo -g 'cat|dog' 'the dog sat on the cat' |
| |
| Note that there is a much more comprehensive test program, called |
| pcretest, which supports many more facilities for testing regular |
| expressions and the PCRE library. The pcredemo program is provided as a |
| simple coding example. |
| |
| If you try to run pcredemo when PCRE is not installed in the standard |
| library directory, you may get an error like this on some operating |
| systems (e.g. Solaris): |
| |
| ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or |
| directory |
| |
| This is caused by the way shared library support works on those sys- |
| tems. You need to add |
| |
| -R/usr/local/lib |
| |
| (for example) to the compile command to get round this problem. |
| |
| |
| AUTHOR |
| |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| |
| REVISION |
| |
| Last updated: 17 November 2010 |
| Copyright (c) 1997-2010 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| PCRELIMITS(3) PCRELIMITS(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions |
| |
| |
| SIZE AND OTHER LIMITATIONS |
| |
| There are some size limitations in PCRE but it is hoped that they will |
| never in practice be relevant. |
| |
| The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE |
| is compiled with the default internal linkage size of 2. If you want to |
| process regular expressions that are truly enormous, you can compile |
| PCRE with an internal linkage size of 3 or 4 (see the README file in |
| the source distribution and the pcrebuild documentation for details). |
| In these cases the limit is substantially larger. However, the speed |
| of execution is slower. |
| |
| All values in repeating quantifiers must be less than 65536. |
| |
| There is no limit to the number of parenthesized subpatterns, but there |
| can be no more than 65535 capturing subpatterns. |
| |
| There is a limit to the number of forward references to subsequent sub- |
| patterns of around 200,000. Repeated forward references with fixed |
| upper limits, for example, (?2){0,100} when subpattern number 2 is to |
| the right, are included in the count. There is no limit to the number |
| of backward references. |
| |
| The maximum length of name for a named subpattern is 32 characters, and |
| the maximum number of named subpatterns is 10000. |
| |
| The maximum length of a subject string is the largest positive number |
| that an integer variable can hold. However, when using the traditional |
| matching function, PCRE uses recursion to handle subpatterns and indef- |
| inite repetition. This means that the available stack space may limit |
| the size of a subject string that can be processed by certain patterns. |
| For a discussion of stack issues, see the pcrestack documentation. |
| |
| |
| AUTHOR |
| |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| |
| REVISION |
| |
| Last updated: 30 November 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| |
| |
| PCRESTACK(3) PCRESTACK(3) |
| |
| |
| NAME |
| PCRE - Perl-compatible regular expressions |
| |
| |
| PCRE DISCUSSION OF STACK USAGE |
| |
| When you call pcre_exec(), it makes use of an internal function called |
| match(). This calls itself recursively at branch points in the pattern, |
| in order to remember the state of the match so that it can back up and |
| try a different alternative if the first one fails. As matching pro- |
| ceeds deeper and deeper into the tree of possibilities, the recursion |
| depth increases. The match() function is also called in other circum- |
| stances, for example, whenever a parenthesized sub-pattern is entered, |
| and in certain cases of repetition. |
| |
| Not all calls of match() increase the recursion depth; for an item such |
| as a* it may be called several times at the same level, after matching |
| different numbers of a's. Furthermore, in a number of cases where the |
| result of the recursive call would immediately be passed back as the |
| result of the current call (a "tail recursion"), the function is just |
| restarted instead. |
| |
| The above comments apply when pcre_exec() is run in its normal inter- |
| pretive manner. If the pattern was studied with the PCRE_STUDY_JIT_COM- |
| PILE option, and just-in-time compiling was successful, and the options |
| passed to pcre_exec() were not incompatible, the matching process uses |
| the JIT-compiled code instead of the match() function. In this case, |
| the memory requirements are handled entirely differently. See the pcre- |
| jit documentation for details. |
| |
| The pcre_dfa_exec() function operates in an entirely different way, and |
| uses recursion only when there is a regular expression recursion or |
| subroutine call in the pattern. This includes the processing of asser- |
| tion and "once-only" subpatterns, which are handled like subroutine |
| calls. Normally, these are never very deep, and the limit on the com- |
| plexity of pcre_dfa_exec() is controlled by the amount of workspace it |
| is given. However, it is possible to write patterns with runaway infi- |
| nite recursions; such patterns will cause pcre_dfa_exec() to run out of |
| stack. At present, there is no protection against this. |
| |
| The comments that follow do NOT apply to pcre_dfa_exec(); they are rel- |
| evant only for pcre_exec() without the JIT optimization. |
| |
| Reducing pcre_exec()'s stack usage |
| |
| Each time that match() is actually called recursively, it uses memory |
| from the process stack. For certain kinds of pattern and data, very |
| large amounts of stack may be needed, despite the recognition of "tail |
| recursion". You can often reduce the amount of recursion, and there- |
| fore the amount of stack used, by modifying the pattern that is being |
| matched. Consider, for example, this pattern: |
| |
| ([^<]|<(?!inet))+ |
| |
| It matches from wherever it starts until it encounters "<inet" or the |
| end of the data, and is the kind of pattern that might be used when |
| processing an XML file. Each iteration of the outer parentheses matches |
| either one character that is not "<" or a "<" that is not followed by |
| "inet". However, each time a parenthesis is processed, a recursion |
| occurs, so this formulation uses a stack frame for each matched charac- |
| ter. For a long string, a lot of stack is required. Consider now this |
| rewritten pattern, which matches exactly the same strings: |
| |
| ([^<]++|<(?!inet))+ |
| |
| This uses very much less stack, because runs of characters that do not |
| contain "<" are "swallowed" in one item inside the parentheses. Recur- |
| sion happens only when a "<" character that is not followed by "inet" |
| is encountered (and we assume this is relatively rare). A possessive |
| quantifier is used to stop any backtracking into the runs of non-"<" |
| characters, but that is not related to stack usage. |
| |
| This example shows that one way of avoiding stack problems when match- |
| ing long subject strings is to write repeated parenthesized subpatterns |
| to match more than one character whenever possible. |
| |
| Compiling PCRE to use heap instead of stack for pcre_exec() |
| |
| In environments where stack memory is constrained, you might want to |
| compile PCRE to use heap memory instead of stack for remembering back- |
| up points when pcre_exec() is running. This makes it run a lot more |
| slowly, however. Details of how to do this are given in the pcrebuild |
| documentation. When built in this way, instead of using the stack, PCRE |
| obtains and frees memory by calling the functions that are pointed to |
| by the pcre_stack_malloc and pcre_stack_free variables. By default, |
| these point to malloc() and free(), but you can replace the pointers to |
| cause PCRE to use your own functions. Since the block sizes are always |
| the same, and are always freed in reverse order, it may be possible to |
| implement customized memory handlers that are more efficient than the |
| standard functions. |
| |
| Limiting pcre_exec()'s stack usage |
| |
| You can set limits on the number of times that match() is called, both |
| in total and recursively. If a limit is exceeded, pcre_exec() returns |
| an error code. Setting suitable limits should prevent it from running |
| out of stack. The default values of the limits are very large, and |
| unlikely ever to operate. They can be changed when PCRE is built, and |
| they can also be set when pcre_exec() is called. For details of these |
| interfaces, see the pcrebuild documentation and the section on extra |
| data for pcre_exec() in the pcreapi documentation. |
| |
| As a very rough rule of thumb, you should reckon on about 500 bytes per |
| recursion. Thus, if you want to limit your stack usage to 8Mb, you |
| should set the limit at 16000 recursions. A 64Mb stack, on the other |
| hand, can support around 128000 recursions. |
| |
| In Unix-like environments, the pcretest test program has a command line |
| option (-S) that can be used to increase the size of its stack. As long |
| as the stack is large enough, another option (-M) can be used to find |
| the smallest limits that allow a particular pattern to match a given |
| subject string. This is done by calling pcre_exec() repeatedly with |
| different limits. |
| |
| Changing stack size in Unix-like systems |
| |
| In Unix-like environments, there is not often a problem with the stack |
| unless very long strings are involved, though the default limit on |
| stack size varies from system to system. Values from 8Mb to 64Mb are |
| common. You can find your default limit by running the command: |
| |
| ulimit -s |
| |
| Unfortunately, the effect of running out of stack is often SIGSEGV, |
| though sometimes a more explicit error message is given. You can nor- |
| mally increase the limit on stack size by code such as this: |
| |
| struct rlimit rlim; |
| getrlimit(RLIMIT_STACK, &rlim); |
| rlim.rlim_cur = 100*1024*1024; |
| setrlimit(RLIMIT_STACK, &rlim); |
| |
| This reads the current limits (soft and hard) using getrlimit(), then |
| attempts to increase the soft limit to 100Mb using setrlimit(). You |
| must do this before calling pcre_exec(). |
| |
| Changing stack size in Mac OS X |
| |
| Using setrlimit(), as described above, should also work on Mac OS X. It |
| is also possible to set a stack size when linking a program. There is a |
| discussion about stack sizes in Mac OS X at this web site: |
| http://developer.apple.com/qa/qa2005/qa1419.html. |
| |
| |
| AUTHOR |
| |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| |
| |
| REVISION |
| |
| Last updated: 26 August 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| ------------------------------------------------------------------------------ |
| |
| |