| .TH PCREPATTERN 3 |
| .SH NAME |
| PCRE - Perl-compatible regular expressions |
| .SH "PCRE REGULAR EXPRESSION DETAILS" |
| .rs |
| .sp |
| The syntax and semantics of the regular expressions that are supported by PCRE |
| are described in detail below. There is a quick-reference syntax summary in the |
| .\" HREF |
| \fBpcresyntax\fP |
| .\" |
| 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 syntax) in order to provide some compatibility with |
| regular expressions in Python, .NET, and Oniguruma. |
| .P |
| 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. |
| .P |
| 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 |
| \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There |
| is also a special sequence that can be given at the start of a pattern: |
| .sp |
| (*UTF8) |
| .sp |
| 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 |
| .\" HREF |
| \fBpcreunicode\fP |
| .\" |
| page. |
| .P |
| Another special sequence that may appear at the start of a pattern or in |
| combination with (*UTF8) is: |
| .sp |
| (*UCP) |
| .sp |
| This has the same effect as setting the PCRE_UCP option: it causes sequences |
| such as \ed and \ew to use Unicode properties to determine character types, |
| instead of recognizing only characters with codes less than 128 via a lookup |
| table. |
| .P |
| 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 concerned with the handling |
| of newlines; they are described below. |
| .P |
| The remainder of this document discusses the patterns that are supported by |
| PCRE when its main matching function, \fBpcre_exec()\fP, is used. |
| From release 6.0, PCRE offers a second matching function, |
| \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not |
| Perl-compatible. Some of the features discussed below are not available when |
| \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the |
| alternative function, and how it differs from the normal function, are |
| discussed in the |
| .\" HREF |
| \fBpcrematching\fP |
| .\" |
| page. |
| . |
| . |
| .\" HTML <a name="newlines"></a> |
| .SH "NEWLINE CONVENTIONS" |
| .rs |
| .sp |
| PCRE supports five different conventions for indicating line breaks in |
| strings: a single CR (carriage return) character, a single LF (linefeed) |
| character, the two-character sequence CRLF, any of the three preceding, or any |
| Unicode newline sequence. The |
| .\" HREF |
| \fBpcreapi\fP |
| .\" |
| page has |
| .\" HTML <a href="pcreapi.html#newlines"> |
| .\" </a> |
| further discussion |
| .\" |
| about newlines, and shows how to set the newline convention in the |
| \fIoptions\fP arguments for the compiling and matching functions. |
| .P |
| It is also possible to specify a newline convention by starting a pattern |
| string with one of the following five sequences: |
| .sp |
| (*CR) carriage return |
| (*LF) linefeed |
| (*CRLF) carriage return, followed by linefeed |
| (*ANYCRLF) any of the three above |
| (*ANY) all Unicode newline sequences |
| .sp |
| These override the default and the options given to \fBpcre_compile()\fP or |
| \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default |
| newline sequence, the pattern |
| .sp |
| (*CR)a.b |
| .sp |
| changes the convention to CR. That pattern matches "a\enb" 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. |
| .P |
| The newline convention affects the interpretation of the dot metacharacter when |
| PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not |
| affect what the \eR escape sequence matches. By default, this is any Unicode |
| newline sequence, for Perl compatibility. However, this can be changed; see the |
| description of \eR in the section entitled |
| .\" HTML <a href="#newlineseq"> |
| .\" </a> |
| "Newline sequences" |
| .\" |
| below. A change of \eR setting can be combined with a change of newline |
| convention. |
| . |
| . |
| .SH "CHARACTERS AND METACHARACTERS" |
| .rs |
| .sp |
| 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 |
| .sp |
| The quick brown fox |
| .sp |
| 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 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 for characters 128 and above, you must |
| ensure that PCRE is compiled with Unicode property support as well as with |
| UTF-8 support. |
| .P |
| 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 |
| \fImetacharacters\fP, which do not stand for themselves but instead are |
| interpreted in some special way. |
| .P |
| There are two different sets of metacharacters: those that are recognized |
| anywhere in the pattern except within square brackets, and those that are |
| recognized within square brackets. Outside square brackets, the metacharacters |
| are as follows: |
| .sp |
| \e 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 |
| .sp |
| Part of a pattern that is in square brackets is called a "character class". In |
| a character class the only metacharacters are: |
| .sp |
| \e general escape character |
| ^ negate the class, but only if the first character |
| - indicates character range |
| .\" JOIN |
| [ POSIX character class (only if followed by POSIX |
| syntax) |
| ] terminates the character class |
| .sp |
| The following sections describe the use of each of the metacharacters. |
| . |
| . |
| .SH BACKSLASH |
| .rs |
| .sp |
| 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. |
| .P |
| For example, if you want to match a * character, you write \e* 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 backslash, you write \e\e. |
| .P |
| 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. |
| .P |
| 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 escaping backslash can |
| be used to include a whitespace or # character as part of the pattern. |
| .P |
| If you want to remove the special meaning from a sequence of characters, you |
| can do so by putting them between \eQ and \eE. This is different from Perl in |
| that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in |
| Perl, $ and @ cause variable interpolation. Note the following examples: |
| .sp |
| Pattern PCRE matches Perl matches |
| .sp |
| .\" JOIN |
| \eQabc$xyz\eE abc$xyz abc followed by the |
| contents of $xyz |
| \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz |
| \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz |
| .sp |
| The \eQ...\eE sequence is recognized both inside and outside character classes. |
| An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed |
| by \eE later in the pattern, the literal interpretation continues to the end of |
| the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside |
| a character class, this causes an error, because the character class is not |
| terminated. |
| . |
| . |
| .\" HTML <a name="digitsafterbackslash"></a> |
| .SS "Non-printing characters" |
| .rs |
| .sp |
| A second use of backslash provides a way of encoding non-printing characters |
| 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: |
| .sp |
| \ea alarm, that is, the BEL character (hex 07) |
| \ecx "control-x", where x is any ASCII character |
| \ee escape (hex 1B) |
| \ef formfeed (hex 0C) |
| \en linefeed (hex 0A) |
| \er carriage return (hex 0D) |
| \et tab (hex 09) |
| \eddd character with octal code ddd, or back reference |
| \exhh character with hex code hh |
| \ex{hhh..} character with hex code hhh.. (non-JavaScript mode) |
| \euhhhh character with hex code hhhh (JavaScript mode only) |
| .sp |
| The precise effect of \ecx 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 \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while |
| \ec; becomes hex 7B (; is 3B). If the byte following \ec 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.) |
| .P |
| By default, after \ex, from zero to two hexadecimal digits are read (letters |
| can be in upper or lower case). Any number of hexadecimal digits may appear |
| between \ex{ 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. |
| .P |
| If characters other than hexadecimal digits appear between \ex{ and }, or if |
| there is no terminating }, this form of escape is not recognized. Instead, the |
| initial \ex will be interpreted as a basic hexadecimal escape, with no |
| following digits, giving a character whose value is zero. |
| .P |
| If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is |
| as just described only when it is followed by two hexadecimal digits. |
| Otherwise, it matches a literal "x" character. In JavaScript mode, support for |
| code points greater than 256 is provided by \eu, which must be followed by |
| four hexadecimal digits; otherwise it matches a literal "u" character. |
| .P |
| Characters whose value is less than 256 can be defined by either of the two |
| syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the |
| way they are handled. For example, \exdc is exactly the same as \ex{dc} (or |
| \eu00dc in JavaScript mode). |
| .P |
| After \e0 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 \e0\ex\e07 |
| 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. |
| .P |
| The handling of a backslash followed by a digit other than 0 is complicated. |
| Outside a character class, PCRE reads it and any following digits 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 \fIback reference\fP. A description of how this works is given |
| .\" HTML <a href="#backreferences"> |
| .\" </a> |
| later, |
| .\" |
| following the discussion of |
| .\" HTML <a href="#subpattern"> |
| .\" </a> |
| parenthesized subpatterns. |
| .\" |
| .P |
| 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 generate 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 \e400. In UTF-8 mode, values up |
| to \e777 are permitted. For example: |
| .sp |
| \e040 is another way of writing a space |
| .\" JOIN |
| \e40 is the same, provided there are fewer than 40 |
| previous capturing subpatterns |
| \e7 is always a back reference |
| .\" JOIN |
| \e11 might be a back reference, or another way of |
| writing a tab |
| \e011 is always a tab |
| \e0113 is a tab followed by the character "3" |
| .\" JOIN |
| \e113 might be a back reference, otherwise the |
| character with octal code 113 |
| .\" JOIN |
| \e377 might be a back reference, otherwise |
| the byte consisting entirely of 1 bits |
| .\" JOIN |
| \e81 is either a back reference, or a binary zero |
| followed by the two characters "8" and "1" |
| .sp |
| 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. |
| .P |
| All the sequences that define a single character value can be used both inside |
| and outside character classes. In addition, inside a character class, \eb is |
| interpreted as the backspace character (hex 08). |
| .P |
| \eN is not allowed in a character class. \eB, \eR, and \eX 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. |
| . |
| . |
| .SS "Unsupported escape sequences" |
| .rs |
| .sp |
| In Perl, the sequences \el, \eL, \eu, and \eU 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, \eU matches a "U" character, and \eu can be used to define a |
| character by code point, as described in the previous section. |
| . |
| . |
| .SS "Absolute and relative back references" |
| .rs |
| .sp |
| The sequence \eg followed by an unsigned or a negative number, optionally |
| enclosed in braces, is an absolute or relative back reference. A named back |
| reference can be coded as \eg{name}. Back references are discussed |
| .\" HTML <a href="#backreferences"> |
| .\" </a> |
| later, |
| .\" |
| following the discussion of |
| .\" HTML <a href="#subpattern"> |
| .\" </a> |
| parenthesized subpatterns. |
| .\" |
| . |
| . |
| .SS "Absolute and relative subroutine calls" |
| .rs |
| .sp |
| For compatibility with Oniguruma, the non-Perl syntax \eg 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 |
| .\" HTML <a href="#onigurumasubroutines"> |
| .\" </a> |
| later. |
| .\" |
| Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP |
| synonymous. The former is a back reference; the latter is a |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </a> |
| subroutine |
| .\" |
| call. |
| . |
| . |
| .\" HTML <a name="genericchartypes"></a> |
| .SS "Generic character types" |
| .rs |
| .sp |
| Another use of backslash is for specifying generic character types: |
| .sp |
| \ed any decimal digit |
| \eD any character that is not a decimal digit |
| \eh any horizontal whitespace character |
| \eH any character that is not a horizontal whitespace character |
| \es any whitespace character |
| \eS any character that is not a whitespace character |
| \ev any vertical whitespace character |
| \eV any character that is not a vertical whitespace character |
| \ew any "word" character |
| \eW any "non-word" character |
| .sp |
| There is also the single sequence \eN, which matches a non-newline character. |
| This is the same as |
| .\" HTML <a href="#fullstopdot"> |
| .\" </a> |
| the "." metacharacter |
| .\" |
| when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name; |
| PCRE does not support this. |
| .P |
| Each pair of lower and upper case escape sequences partitions the complete 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. |
| .P |
| For compatibility with Perl, \es does not match the VT character (code 11). |
| This makes it different from the the POSIX "space" class. The \es characters |
| are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is |
| included in a Perl script, \es may match the VT character. In PCRE, it never |
| does. |
| .P |
| A "word" character is an underscore or any character that is a letter or digit. |
| By default, the definition of letters and digits is controlled by PCRE's |
| low-valued character tables, and may vary if locale-specific matching is taking |
| place (see |
| .\" HTML <a href="pcreapi.html#localesupport"> |
| .\" </a> |
| "Locale support" |
| .\" |
| in the |
| .\" HREF |
| \fBpcreapi\fP |
| .\" |
| 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 \ew. The use of locales with |
| Unicode is discouraged. |
| .P |
| By default, in UTF-8 mode, characters with values greater than 128 never match |
| \ed, \es, or \ew, and always match \eD, \eS, and \eW. 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 behaviour is changed so that Unicode |
| properties are used to determine character types, as follows: |
| .sp |
| \ed any character that \ep{Nd} matches (decimal digit) |
| \es any character that \ep{Z} matches, plus HT, LF, FF, CR |
| \ew any character that \ep{L} or \ep{N} matches, plus underscore |
| .sp |
| The upper case escapes match the inverse sets of characters. Note that \ed |
| matches only decimal digits, whereas \ew matches any Unicode digit, as well as |
| any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and |
| \eB because they are defined in terms of \ew and \eW. Matching these sequences |
| is noticeably slower when PCRE_UCP is set. |
| .P |
| The sequences \eh, \eH, \ev, and \eV 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 horizontal space characters |
| are: |
| .sp |
| 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 |
| .sp |
| The vertical space characters are: |
| .sp |
| 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 |
| . |
| . |
| .\" HTML <a name="newlineseq"></a> |
| .SS "Newline sequences" |
| .rs |
| .sp |
| Outside a character class, by default, the escape sequence \eR matches any |
| Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following: |
| .sp |
| (?>\er\en|\en|\ex0b|\ef|\er|\ex85) |
| .sp |
| This is an example of an "atomic group", details of which are given |
| .\" HTML <a href="#atomicgroup"> |
| .\" </a> |
| 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. |
| .P |
| In UTF-8 mode, two additional characters whose codepoints are greater than 255 |
| are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029). |
| Unicode character property support is not needed for these characters to be |
| recognized. |
| .P |
| It is possible to restrict \eR 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: |
| .sp |
| (*BSR_ANYCRLF) CR, LF, or CRLF only |
| (*BSR_UNICODE) any Unicode newline sequence |
| .sp |
| These override the default and the options given to \fBpcre_compile()\fP or |
| \fBpcre_compile2()\fP, but they can be overridden by options given to |
| \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. 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: |
| .sp |
| (*ANY)(*BSR_ANYCRLF) |
| .sp |
| They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside |
| a character class, \eR is treated as an unrecognized escape sequence, and so |
| matches the letter "R" by default, but causes an error if PCRE_EXTRA is set. |
| . |
| . |
| .\" HTML <a name="uniextseq"></a> |
| .SS Unicode character properties |
| .rs |
| .sp |
| When PCRE is built with Unicode character property support, three additional |
| 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: |
| .sp |
| \ep{\fIxx\fP} a character with the \fIxx\fP property |
| \eP{\fIxx\fP} a character without the \fIxx\fP property |
| \eX an extended Unicode sequence |
| .sp |
| The property names represented by \fIxx\fP 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 |
| .\" HTML <a href="#extraprops"> |
| .\" </a> |
| next section). |
| .\" |
| Other Perl properties such as "InMusicalSymbols" are not currently supported by |
| PCRE. Note that \eP{Any} does not match any characters, so always causes a |
| match failure. |
| .P |
| 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: |
| .sp |
| \ep{Greek} |
| \eP{Han} |
| .sp |
| Those that are not part of an identified script are lumped together as |
| "Common". The current list of scripts is: |
| .P |
| Arabic, |
| Armenian, |
| Avestan, |
| Balinese, |
| Bamum, |
| Bengali, |
| Bopomofo, |
| Braille, |
| Buginese, |
| Buhid, |
| Canadian_Aboriginal, |
| Carian, |
| Cham, |
| Cherokee, |
| Common, |
| Coptic, |
| Cuneiform, |
| Cypriot, |
| Cyrillic, |
| Deseret, |
| Devanagari, |
| Egyptian_Hieroglyphs, |
| Ethiopic, |
| Georgian, |
| Glagolitic, |
| Gothic, |
| Greek, |
| Gujarati, |
| Gurmukhi, |
| Han, |
| Hangul, |
| Hanunoo, |
| Hebrew, |
| Hiragana, |
| Imperial_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. |
| .P |
| Each character has exactly one Unicode general category property, specified by |
| a two-letter abbreviation. For compatibility with Perl, negation can be |
| specified by including a circumflex between the opening brace and the property |
| name. For example, \ep{^Lu} is the same as \eP{Lu}. |
| .P |
| If only one letter is specified with \ep or \eP, it includes all the general |
| 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: |
| .sp |
| \ep{L} |
| \epL |
| .sp |
| The following general category property codes are supported: |
| .sp |
| C Other |
| Cc Control |
| Cf Format |
| Cn Unassigned |
| Co Private use |
| Cs Surrogate |
| .sp |
| L Letter |
| Ll Lower case letter |
| Lm Modifier letter |
| Lo Other letter |
| Lt Title case letter |
| Lu Upper case letter |
| .sp |
| M Mark |
| Mc Spacing mark |
| Me Enclosing mark |
| Mn Non-spacing mark |
| .sp |
| N Number |
| Nd Decimal number |
| Nl Letter number |
| No Other number |
| .sp |
| P Punctuation |
| Pc Connector punctuation |
| Pd Dash punctuation |
| Pe Close punctuation |
| Pf Final punctuation |
| Pi Initial punctuation |
| Po Other punctuation |
| Ps Open punctuation |
| .sp |
| S Symbol |
| Sc Currency symbol |
| Sk Modifier symbol |
| Sm Mathematical symbol |
| So Other symbol |
| .sp |
| Z Separator |
| Zl Line separator |
| Zp Paragraph separator |
| Zs Space separator |
| .sp |
| 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". |
| .P |
| 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 checking has been turned off |
| (see the discussion of PCRE_NO_UTF8_CHECK in the |
| .\" HREF |
| \fBpcreapi\fP |
| .\" |
| page). Perl does not support the Cs property. |
| .P |
| The long synonyms for property names that Perl supports (such as \ep{Letter}) |
| are not supported by PCRE, nor is it permitted to prefix any of these |
| properties with "Is". |
| .P |
| No character that is in the Unicode table has the Cn (unassigned) property. |
| Instead, this property is assumed for any code point that is not in the |
| Unicode table. |
| .P |
| Specifying caseless matching does not affect these escape sequences. For |
| example, \ep{Lu} always matches only upper case letters. |
| .P |
| The \eX escape matches any number of Unicode characters that form an extended |
| Unicode sequence. \eX is equivalent to |
| .sp |
| (?>\ePM\epM*) |
| .sp |
| 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 |
| .\" HTML <a href="#atomicgroup"> |
| .\" </a> |
| (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 \eX matches any one character. |
| .P |
| Note that recent versions of Perl have changed \eX to match what Unicode calls |
| an "extended grapheme cluster", which has a more complicated definition. |
| .P |
| 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 \ed and \ew do not use Unicode |
| properties in PCRE by default, though you can make them do so by setting the |
| PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with |
| (*UCP). |
| . |
| . |
| .\" HTML <a name="extraprops"></a> |
| .SS PCRE's additional properties |
| .rs |
| .sp |
| As well as the standard Unicode properties described in the previous |
| section, PCRE supports four more that make it possible to convert traditional |
| escape sequences such as \ew and \es and POSIX character classes to use Unicode |
| properties. PCRE uses these non-standard, non-Perl properties internally when |
| PCRE_UCP is set. They are: |
| .sp |
| Xan Any alphanumeric character |
| Xps Any POSIX space character |
| Xsp Any Perl space character |
| Xwd Any Perl "word" character |
| .sp |
| Xan matches characters that have either the L (letter) or the N (number) |
| 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. |
| . |
| . |
| .\" HTML <a name="resetmatchstart"></a> |
| .SS "Resetting the match start" |
| .rs |
| .sp |
| The escape sequence \eK causes any previously matched characters not to be |
| included in the final matched sequence. For example, the pattern: |
| .sp |
| foo\eKbar |
| .sp |
| matches "foobar", but reports that it has matched "bar". This feature is |
| similar to a lookbehind assertion |
| .\" HTML <a href="#lookbehind"> |
| .\" </a> |
| (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 \eK does |
| not interfere with the setting of |
| .\" HTML <a href="#subpattern"> |
| .\" </a> |
| captured substrings. |
| .\" |
| For example, when the pattern |
| .sp |
| (foo)\eKbar |
| .sp |
| matches "foobar", the first substring is still set to "foo". |
| .P |
| Perl documents that the use of \eK within assertions is "not well defined". In |
| PCRE, \eK is acted upon when it occurs inside positive assertions, but is |
| ignored in negative assertions. |
| . |
| . |
| .\" HTML <a name="smallassertions"></a> |
| .SS "Simple assertions" |
| .rs |
| .sp |
| The final use of backslash is for certain simple assertions. An assertion |
| 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 |
| .\" HTML <a href="#bigassertions"> |
| .\" </a> |
| below. |
| .\" |
| The backslashed assertions are: |
| .sp |
| \eb matches at a word boundary |
| \eB matches when not at a word boundary |
| \eA matches at the start of the subject |
| \eZ matches at the end of the subject |
| also matches before a newline at the end of the subject |
| \ez matches only at the end of the subject |
| \eG matches at the first matching position in the subject |
| .sp |
| Inside a character class, \eb 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 character (for example, \eB |
| matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid |
| escape sequence" error is generated instead. |
| .P |
| A word boundary is a position in the subject string where the current character |
| and the previous character do not both match \ew or \eW (i.e. one matches |
| \ew and the other matches \eW), or the start or end of the string if the |
| first or last character matches \ew, respectively. In UTF-8 mode, the meanings |
| of \ew and \eW can be changed by setting the PCRE_UCP option. When this is |
| done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start |
| of word" or "end of word" metasequence. However, whatever follows \eb normally |
| determines which it is. For example, the fragment \eba matches "a" at the start |
| of a word. |
| .P |
| The \eA, \eZ, and \ez 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 assertions 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 \fIstartoffset\fP |
| argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start |
| at a point other than the beginning of the subject, \eA can never match. The |
| difference between \eZ and \ez is that \eZ matches before a newline at the end |
| of the string as well as at the very end, whereas \ez matches only at the end. |
| .P |
| The \eG assertion is true only when the current matching position is at the |
| start point of the match, as specified by the \fIstartoffset\fP argument of |
| \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is |
| non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate |
| arguments, you can mimic Perl's /g option, and it is in this kind of |
| implementation where \eG can be useful. |
| .P |
| Note, however, that PCRE's interpretation of \eG, 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. |
| .P |
| If all the alternatives of a pattern begin with \eG, the expression is anchored |
| to the starting match position, and the "anchored" flag is set in the compiled |
| regular expression. |
| . |
| . |
| .SH "CIRCUMFLEX AND DOLLAR" |
| .rs |
| .sp |
| 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 \fIstartoffset\fP argument of |
| \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE |
| option is unset. Inside a character class, circumflex has an entirely different |
| meaning |
| .\" HTML <a href="#characterclass"> |
| .\" </a> |
| (see below). |
| .\" |
| .P |
| 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 subject, it is said to be an |
| "anchored" pattern. (There are also other constructs that can cause a pattern |
| to be anchored.) |
| .P |
| 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. |
| .P |
| 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 \eZ assertion. |
| .P |
| 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. |
| .P |
| For example, the pattern /^abc$/ matches the subject string "def\enabc" (where |
| \en 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 \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The |
| PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. |
| .P |
| Note that the sequences \eA, \eZ, and \ez can be used to match the start and |
| end of the subject in both modes, and if all branches of a pattern start with |
| \eA it is always anchored, whether or not PCRE_MULTILINE is set. |
| . |
| . |
| .\" HTML <a name="fullstopdot"></a> |
| .SH "FULL STOP (PERIOD, DOT) AND \eN" |
| .rs |
| .sp |
| Outside a character class, a dot in the pattern matches any one character in |
| the subject string except (by default) a character that signifies the end of a |
| line. In UTF-8 mode, the matched character may be more than one byte long. |
| .P |
| 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 Unicode line endings are being |
| recognized, dot does not match CR or LF or any of the other line ending |
| characters. |
| .P |
| 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. |
| .P |
| The handling of dot is entirely independent of the handling of circumflex and |
| dollar, the only relationship being that they both involve newlines. Dot has no |
| special meaning in a character class. |
| .P |
| The escape sequence \eN 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 \eN to match characters by |
| name; PCRE does not support this. |
| . |
| . |
| .SH "MATCHING A SINGLE BYTE" |
| .rs |
| .sp |
| Outside a character class, the escape sequence \eC 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 \eC |
| breaks up characters into individual bytes, matching one byte with \eC 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). |
| .P |
| PCRE does not allow \eC to appear in lookbehind assertions |
| .\" HTML <a href="#lookbehind"> |
| .\" </a> |
| (described below) |
| .\" |
| in UTF-8 mode, because this would make it impossible to calculate the length of |
| the lookbehind. |
| .P |
| In general, the \eC escape sequence is best avoided in UTF-8 mode. However, 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 character, as in this pattern |
| (ignore white space and line breaks): |
| .sp |
| (?| (?=[\ex00-\ex7f])(\eC) | |
| (?=[\ex80-\ex{7ff}])(\eC)(\eC) | |
| (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) | |
| (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC)) |
| .sp |
| A group that starts with (?| resets the capturing parentheses numbers in each |
| alternative (see |
| .\" HTML <a href="#dupsubpatternnumber"> |
| .\" </a> |
| "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 number of |
| groups. |
| . |
| . |
| .\" HTML <a name="characterclass"></a> |
| .SH "SQUARE BRACKETS AND CHARACTER CLASSES" |
| .rs |
| .sp |
| An opening square bracket introduces a character class, terminated by a closing |
| square bracket. A closing square bracket on its own is not special 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. |
| .P |
| 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. |
| .P |
| 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 consumes a character from the subject |
| string, and therefore it fails if the current pointer is at the end of the |
| string. |
| .P |
| 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 \ex{ escaping mechanism. |
| .P |
| 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. |
| .P |
| 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. |
| .P |
| The minus (hyphen) character can be used to specify a range of characters 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. |
| .P |
| It is not possible to have the literal character "]" as the end character 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-\e]46] is interpreted 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. |
| .P |
| Ranges operate in the collating sequence of character values. They can also be |
| used for characters specified numerically, for example [\e000-\e037]. In UTF-8 |
| mode, ranges can include characters whose values are greater than 255, for |
| example [\ex{100}-\ex{2ff}]. |
| .P |
| 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 |
| [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character |
| tables for a French locale are in use, [\exc8-\excb] 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. |
| .P |
| The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, |
| \eV, \ew, and \eW may appear in a character class, and add the characters that |
| they match to the class. For example, [\edABCDEF] matches any hexadecimal |
| digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew |
| and their upper case partners, just as it does when they appear outside a |
| character class, as described in the section entitled |
| .\" HTML <a href="#genericchartypes"> |
| .\" </a> |
| "Generic character types" |
| .\" |
| above. The escape sequence \eb has a different meaning inside a character |
| class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX |
| 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. |
| .P |
| 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 [^\eW_] matches any letter or digit, but not underscore, |
| whereas [\ew] 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 ...". |
| .P |
| 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. |
| . |
| . |
| .SH "POSIX CHARACTER CLASSES" |
| .rs |
| .sp |
| 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, |
| .sp |
| [01[:alpha:]%] |
| .sp |
| matches "0", "1", any alphabetic character, or "%". The supported class names |
| are: |
| .sp |
| alnum letters and digits |
| alpha letters |
| ascii character codes 0 - 127 |
| blank space or tab only |
| cntrl control characters |
| digit decimal digits (same as \ed) |
| 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 \es) |
| upper upper case letters |
| word "word" characters (same as \ew) |
| xdigit hexadecimal digits |
| .sp |
| 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 \es, which does not include VT (for Perl |
| compatibility). |
| .P |
| 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, |
| .sp |
| [12[:^digit:]] |
| .sp |
| 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. |
| .P |
| 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 \fBpcre_compile()\fP, some of the classes are changed so that Unicode |
| character properties are used. This is achieved by replacing the POSIX classes |
| by other sequences, as follows: |
| .sp |
| [:alnum:] becomes \ep{Xan} |
| [:alpha:] becomes \ep{L} |
| [:blank:] becomes \eh |
| [:digit:] becomes \ep{Nd} |
| [:lower:] becomes \ep{Ll} |
| [:space:] becomes \ep{Xps} |
| [:upper:] becomes \ep{Lu} |
| [:word:] becomes \ep{Xwd} |
| .sp |
| Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX |
| classes are unchanged, and match only characters with code points less than |
| 128. |
| . |
| . |
| .SH "VERTICAL BAR" |
| .rs |
| .sp |
| Vertical bar characters are used to separate alternative patterns. For example, |
| the pattern |
| .sp |
| gilbert|sullivan |
| .sp |
| 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 |
| .\" HTML <a href="#subpattern"> |
| .\" </a> |
| (defined below), |
| .\" |
| "succeeds" means matching the rest of the main pattern as well as the |
| alternative in the subpattern. |
| . |
| . |
| .SH "INTERNAL OPTION SETTING" |
| .rs |
| .sp |
| 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 |
| .sp |
| i for PCRE_CASELESS |
| m for PCRE_MULTILINE |
| s for PCRE_DOTALL |
| x for PCRE_EXTENDED |
| .sp |
| For example, (?im) sets caseless, multiline matching. It is also possible to |
| unset these options by preceding the letter with a hyphen, and a combined |
| setting and unsetting such as (?im-sx), which sets PCRE_CASELESS 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. |
| .P |
| 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. |
| .P |
| 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 therefore show up in data |
| extracted by the \fBpcre_fullinfo()\fP function). |
| .P |
| An option change within a subpattern (see below for a description of |
| subpatterns) affects only that part of the subpattern that follows it, so |
| .sp |
| (a(?i)b)c |
| .sp |
| 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, |
| .sp |
| (a(?i)b|c) |
| .sp |
| 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. |
| .P |
| \fBNote:\fP 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 |
| .\" HTML <a href="#newlineseq"> |
| .\" </a> |
| "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. |
| . |
| . |
| .\" HTML <a name="subpattern"></a> |
| .SH SUBPATTERNS |
| .rs |
| .sp |
| Subpatterns are delimited by parentheses (round brackets), which can be nested. |
| Turning part of a pattern into a subpattern does two things: |
| .sp |
| 1. It localizes a set of alternatives. For example, the pattern |
| .sp |
| cat(aract|erpillar|) |
| .sp |
| matches "cataract", "caterpillar", or "cat". Without the parentheses, it would |
| match "cataract", "erpillar" or an empty string. |
| .sp |
| 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 \fIovector\fP argument of |
| \fBpcre_exec()\fP. 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 |
| .sp |
| the ((red|white) (king|queen)) |
| .sp |
| the captured substrings are "red king", "red", and "king", and are numbered 1, |
| 2, and 3, respectively. |
| .P |
| 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 capturing, 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 |
| .sp |
| the ((?:red|white) (king|queen)) |
| .sp |
| the captured substrings are "white queen" and "queen", and are numbered 1 and |
| 2. The maximum number of capturing subpatterns is 65535. |
| .P |
| 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 |
| .sp |
| (?i:saturday|sunday) |
| (?:(?i)saturday|sunday) |
| .sp |
| 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". |
| . |
| . |
| .\" HTML <a name="dupsubpatternnumber"></a> |
| .SH "DUPLICATE SUBPATTERN NUMBERS" |
| .rs |
| .sp |
| 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: |
| .sp |
| (?|(Sat)ur|(Sun))day |
| .sp |
| Because the two alternatives are inside a (?| group, both sets of capturing |
| 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, parentheses 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 following example is taken from the Perl documentation. The |
| numbers underneath show in which buffer the captured content will be stored. |
| .sp |
| # before ---------------branch-reset----------- after |
| / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x |
| # 1 2 2 3 2 3 4 |
| .sp |
| 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": |
| .sp |
| /(?|(abc)|(def))\e1/ |
| .sp |
| 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": |
| .sp |
| /(?|(abc)|(def))(?1)/ |
| .sp |
| If a |
| .\" HTML <a href="#conditions"> |
| .\" </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 number have matched. |
| .P |
| An alternative approach to using this "branch reset" feature is to use |
| duplicate named subpatterns, as described in the next section. |
| . |
| . |
| .SH "NAMED SUBPATTERNS" |
| .rs |
| .sp |
| Identifying capturing parentheses by number is simple, but it can be very hard |
| to keep track of the numbers in complicated regular expressions. Furthermore, |
| if an expression is modified, the numbers may change. To help with this |
| difficulty, PCRE supports the naming of subpatterns. 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 syntax. Perl allows identically numbered subpatterns to |
| have different names, but PCRE does not. |
| .P |
| 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 |
| .\" HTML <a href="#backreferences"> |
| .\" </a> |
| back references, |
| .\" |
| .\" HTML <a href="#recursion"> |
| .\" </a> |
| recursion, |
| .\" |
| and |
| .\" HTML <a href="#conditions"> |
| .\" </a> |
| conditions, |
| .\" |
| can be made by name as well as by number. |
| .P |
| 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. |
| .P |
| 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.) Duplicate 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: |
| .sp |
| (?<DN>Mon|Fri|Sun)(?:day)?| |
| (?<DN>Tue)(?:sday)?| |
| (?<DN>Wed)(?:nesday)?| |
| (?<DN>Thu)(?:rsday)?| |
| (?<DN>Sat)(?:urday)? |
| .sp |
| 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.) |
| .P |
| 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. |
| .P |
| If you make a back reference to a non-unique named subpattern from elsewhere in |
| the pattern, the one that corresponds to the first occurrence 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 |
| .\" |
| .\" HTML <a href="#conditions"> |
| .\" </a> |
| 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 |
| .\" HREF |
| \fBpcreapi\fP |
| .\" |
| documentation. |
| .P |
| \fBWarning:\fP You cannot use different names to distinguish between two |
| subpatterns with the same number because PCRE uses only the numbers when |
| matching. For this reason, an error is given at compile time if different 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. |
| . |
| . |
| .SH REPETITION |
| .rs |
| .sp |
| Repetition is specified by quantifiers, which can follow any of the following |
| items: |
| .sp |
| a literal data character |
| the dot metacharacter |
| the \eC escape sequence |
| the \eX escape sequence (in UTF-8 mode with Unicode properties) |
| the \eR escape sequence |
| an escape such as \ed or \epL 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) |
| .sp |
| The general repetition quantifier specifies a minimum and maximum number 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: |
| .sp |
| z{2,4} |
| .sp |
| 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 |
| .sp |
| [aeiou]{3,} |
| .sp |
| matches at least 3 successive vowels, but may match many more, while |
| .sp |
| \ed{8} |
| .sp |
| 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 example, {,6} is not a |
| quantifier, but a literal string of four characters. |
| .P |
| In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual |
| bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of |
| which is represented by a two-byte sequence. Similarly, when Unicode property |
| support is available, \eX{3} matches three Unicode extended sequences, each of |
| which may be several bytes long (and they may be of different lengths). |
| .P |
| The quantifier {0} is permitted, causing the expression to behave as if the |
| previous item and the quantifier were not present. This may be useful for |
| subpatterns that are referenced as |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </a> |
| subroutines |
| .\" |
| from elsewhere in the pattern (but see also the section entitled |
| .\" HTML <a href="#subdefine"> |
| .\" </a> |
| "Defining subpatterns for use by reference only" |
| .\" |
| below). Items other than subpatterns that have a {0} quantifier are omitted |
| from the compiled pattern. |
| .P |
| For convenience, the three most common quantifiers have single-character |
| abbreviations: |
| .sp |
| * is equivalent to {0,} |
| + is equivalent to {1,} |
| ? is equivalent to {0,1} |
| .sp |
| 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: |
| .sp |
| (a?)* |
| .sp |
| 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 broken. |
| .P |
| 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 |
| .sp |
| /\e*.*\e*/ |
| .sp |
| to the string |
| .sp |
| /* first comment */ not comment /* second comment */ |
| .sp |
| fails, because it matches the entire string owing to the greediness of the .* |
| item. |
| .P |
| 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 |
| .sp |
| /\e*.*?\e*/ |
| .sp |
| 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 |
| .sp |
| \ed??\ed |
| .sp |
| which matches one digit by preference, but can match two if that is the only |
| way the rest of the pattern matches. |
| .P |
| 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. |
| .P |
| 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. |
| .P |
| If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent |
| 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 \eA. |
| .P |
| In cases where it is known that the subject string contains no newlines, it is |
| worth setting PCRE_DOTALL in order to obtain this optimization, or |
| alternatively using ^ to indicate anchoring explicitly. |
| .P |
| 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: |
| .sp |
| (.*)abc\e1 |
| .sp |
| If the subject is "xyz123abc123" the match point is the fourth character. For |
| this reason, such a pattern is not implicitly anchored. |
| .P |
| When a capturing subpattern is repeated, the value captured is the substring |
| that matched the final iteration. For example, after |
| .sp |
| (tweedle[dume]{3}\es*)+ |
| .sp |
| 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 iterations. For |
| example, after |
| .sp |
| /(a|(b))+/ |
| .sp |
| matches "aba" the value of the second captured substring is "b". |
| . |
| . |
| .\" HTML <a name="atomicgroup"></a> |
| .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS" |
| .rs |
| .sp |
| 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. |
| .P |
| Consider, for example, the pattern \ed+foo when applied to the subject line |
| .sp |
| 123456bar |
| .sp |
| 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 \ed+ |
| 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. |
| .P |
| 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: |
| .sp |
| (?>\ed+)foo |
| .sp |
| This kind of parenthesis "locks up" the part of the pattern it contains 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. |
| .P |
| 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. |
| .P |
| 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 \ed+ and \ed+? are prepared to adjust the |
| number of digits they match in order to make the rest of the pattern match, |
| (?>\ed+) can only match an entire sequence of digits. |
| .P |
| 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 |
| .sp |
| \ed++foo |
| .sp |
| Note that a possessive quantifier can be used with an entire group, for |
| example: |
| .sp |
| (abc|xyz){2,3}+ |
| .sp |
| 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. |
| .P |
| The possessive quantifier syntax is an extension to the Perl 5.8 syntax. |
| 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. |
| .P |
| PCRE has an optimization that automatically "possessifies" certain simple |
| 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. |
| .P |
| 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 |
| .sp |
| (\eD+|<\ed+>)*[!?] |
| .sp |
| 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 |
| .sp |
| aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa |
| .sp |
| it takes a long time before reporting failure. This is because the string can |
| be divided between the internal \eD+ 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 character 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: |
| .sp |
| ((?>\eD+)|<\ed+>)*[!?] |
| .sp |
| sequences of non-digits cannot be broken, and failure happens quickly. |
| . |
| . |
| .\" HTML <a name="backreferences"></a> |
| .SH "BACK REFERENCES" |
| .rs |
| .sp |
| Outside a character class, a backslash followed by a digit greater than 0 (and |
| possibly further digits) is a back reference to a capturing subpattern earlier |
| (that is, to its left) in the pattern, provided there have been that many |
| previous capturing left parentheses. |
| .P |
| 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 pattern. 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 iteration. |
| .P |
| 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 \e50 is |
| interpreted as a character defined in octal. See the subsection entitled |
| "Non-printing characters" |
| .\" HTML <a href="#digitsafterbackslash"> |
| .\" </a> |
| 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). |
| .P |
| Another way of avoiding the ambiguity inherent in the use of digits following a |
| backslash is to use the \eg escape sequence. This escape must be followed by an |
| unsigned number or a negative number, optionally enclosed in braces. These |
| examples are all identical: |
| .sp |
| (ring), \e1 |
| (ring), \eg1 |
| (ring), \eg{1} |
| .sp |
| An unsigned number specifies an absolute reference without the ambiguity 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: |
| .sp |
| (abc(def)ghi)\eg{-1} |
| .sp |
| The sequence \eg{-1} is a reference to the most recently started capturing |
| subpattern before \eg, that is, is it equivalent to \e2 in this example. |
| Similarly, \eg{-2} would be equivalent to \e1. 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. |
| .P |
| A back reference matches whatever actually matched the capturing subpattern in |
| the current subject string, rather than anything matching the subpattern |
| itself (see |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </a> |
| "Subpatterns as subroutines" |
| .\" |
| below for a way of doing that). So the pattern |
| .sp |
| (sens|respons)e and \e1ibility |
| .sp |
| 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 example, |
| .sp |
| ((?i)rah)\es+\e1 |
| .sp |
| matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original |
| capturing subpattern is matched caselessly. |
| .P |
| There are several different ways of writing back references to named |
| subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or |
| \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified |
| back reference syntax, in which \eg can be used for both numeric and named |
| references, is also supported. We could rewrite the above example in any of |
| the following ways: |
| .sp |
| (?<p1>(?i)rah)\es+\ek<p1> |
| (?'p1'(?i)rah)\es+\ek{p1} |
| (?P<p1>(?i)rah)\es+(?P=p1) |
| (?<p1>(?i)rah)\es+\eg{p1} |
| .sp |
| A subpattern that is referenced by name may appear in the pattern before or |
| after the reference. |
| .P |
| 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 |
| .sp |
| (a|(bc))\e2 |
| .sp |
| 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 reference to an |
| unset value matches an empty string. |
| .P |
| Because there may be many capturing parentheses in a pattern, all digits |
| following a backslash are taken as part of a potential back reference 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 \eg{ syntax or an empty comment (see |
| .\" HTML <a href="#comments"> |
| .\" </a> |
| "Comments" |
| .\" |
| below) can be used. |
| . |
| .SS "Recursive back references" |
| .rs |
| .sp |
| A back reference that occurs inside the parentheses to which it refers fails |
| when the subpattern is first used, so, for example, (a\e1) never matches. |
| However, such references can be useful inside repeated subpatterns. For |
| example, the pattern |
| .sp |
| (a|b\e1)+ |
| .sp |
| matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration 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. |
| .P |
| Back references of this type cause the group that they reference to be treated |
| as an |
| .\" HTML <a href="#atomicgroup"> |
| .\" </a> |
| atomic group. |
| .\" |
| Once the whole group has been matched, a subsequent matching failure cannot |
| cause backtracking into the middle of the group. |
| . |
| . |
| .\" HTML <a name="bigassertions"></a> |
| .SH ASSERTIONS |
| .rs |
| .sp |
| 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 \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described |
| .\" HTML <a href="#smallassertions"> |
| .\" </a> |
| above. |
| .\" |
| .P |
| 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. |
| .P |
| Assertion subpatterns are not capturing subpatterns. If such an assertion |
| contains capturing subpatterns within it, these are counted for the purposes of |
| numbering the capturing subpatterns in the whole pattern. However, substring |
| capturing is carried out only for positive assertions, because it does not make |
| sense for negative assertions. |
| .P |
| 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: |
| .sp |
| (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 |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </a> |
| subroutine mechanism. |
| .\" |
| .sp |
| (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 greediness of the quantifier. |
| .sp |
| (3) If the minimum repetition is greater than zero, the quantifier is ignored. |
| The assertion is obeyed just once when encountered during matching. |
| . |
| . |
| .SS "Lookahead assertions" |
| .rs |
| .sp |
| Lookahead assertions start with (?= for positive assertions and (?! for |
| negative assertions. For example, |
| .sp |
| \ew+(?=;) |
| .sp |
| matches a word followed by a semicolon, but does not include the semicolon in |
| the match, and |
| .sp |
| foo(?!bar) |
| .sp |
| matches any occurrence of "foo" that is not followed by "bar". Note that the |
| apparently similar pattern |
| .sp |
| (?!foo)bar |
| .sp |
| 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. |
| .P |
| 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 (?!). |
| . |
| . |
| .\" HTML <a name="lookbehind"></a> |
| .SS "Lookbehind assertions" |
| .rs |
| .sp |
| Lookbehind assertions start with (?<= for positive assertions and (?<! for |
| negative assertions. For example, |
| .sp |
| (?<!foo)bar |
| .sp |
| 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 several top-level alternatives, they |
| do not all have to have the same fixed length. Thus |
| .sp |
| (?<=bullock|donkey) |
| .sp |
| is permitted, but |
| .sp |
| (?<!dogs?|cats?) |
| .sp |
| 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 |
| .sp |
| (?<=ab(c|de)) |
| .sp |
| 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: |
| .sp |
| (?<=abc|abde) |
| .sp |
| In some cases, the escape sequence \eK |
| .\" HTML <a href="#resetmatchstart"> |
| .\" </a> |
| (see above) |
| .\" |
| can be used instead of a lookbehind assertion to get round the fixed-length |
| restriction. |
| .P |
| 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 current position, the |
| assertion fails. |
| .P |
| In UTF-8 mode, PCRE does not allow the \eC escape (which matches a single byte, |
| even in UTF-8 mode) to appear in lookbehind assertions, because it makes it |
| impossible to calculate the length of the lookbehind. The \eX and \eR escapes, |
| which can match different numbers of bytes, are also not permitted. |
| .P |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </a> |
| "Subroutine" |
| .\" |
| calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long |
| as the subpattern matches a fixed-length string. |
| .\" HTML <a href="#recursion"> |
| .\" </a> |
| Recursion, |
| .\" |
| however, is not supported. |
| .P |
| 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 |
| .sp |
| abcd$ |
| .sp |
| 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 |
| .sp |
| ^.*abcd$ |
| .sp |
| 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 |
| .sp |
| ^.*+(?<=abcd) |
| .sp |
| 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. |
| . |
| . |
| .SS "Using multiple assertions" |
| .rs |
| .sp |
| Several assertions (of any sort) may occur in succession. For example, |
| .sp |
| (?<=\ed{3})(?<!999)foo |
| .sp |
| 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 \fInot\fP match "foo" preceded by six characters, the first |
| of which are digits and the last three of which are not "999". For example, it |
| doesn't match "123abcfoo". A pattern to do that is |
| .sp |
| (?<=\ed{3}...)(?<!999)foo |
| .sp |
| 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". |
| .P |
| Assertions can be nested in any combination. For example, |
| .sp |
| (?<=(?<!foo)bar)baz |
| .sp |
| matches an occurrence of "baz" that is preceded by "bar" which in turn is not |
| preceded by "foo", while |
| .sp |
| (?<=\ed{3}(?!999)...)foo |
| .sp |
| is another pattern that matches "foo" preceded by three digits and any three |
| characters that are not "999". |
| . |
| . |
| .\" HTML <a name="conditions"></a> |
| .SH "CONDITIONAL SUBPATTERNS" |
| .rs |
| .sp |
| It is possible to cause the matching process to obey a subpattern |
| conditionally or to choose between two alternative subpatterns, depending on |
| the result of an assertion, or whether a specific capturing subpattern has |
| already been matched. The two possible forms of conditional subpattern are: |
| .sp |
| (?(condition)yes-pattern) |
| (?(condition)yes-pattern|no-pattern) |
| .sp |
| If the condition is satisfied, the yes-pattern is used; otherwise the |
| no-pattern (if present) is used. If there are more than two alternatives in the |
| subpattern, a compile-time error occurs. Each of the two alternatives may |
| itself contain nested subpatterns of any form, including 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: |
| .sp |
| (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) |
| .sp |
| .P |
| There are four kinds of condition: references to subpatterns, references to |
| recursion, a pseudo-condition called DEFINE, and assertions. |
| . |
| .SS "Checking for a used subpattern by number" |
| .rs |
| .sp |
| If the text between the parentheses consists of a sequence of digits, the |
| condition is true if a capturing subpattern of that number has previously |
| matched. If there is more than one capturing subpattern with the same number |
| (see the earlier |
| .\" |
| .\" HTML <a href="#recursion"> |
| .\" </a> |
| section about duplicate subpattern numbers), |
| .\" |
| the condition is true if any of them have matched. An alternative 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.) |
| .P |
| 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: |
| .sp |
| ( \e( )? [^()]+ (?(1) \e) ) |
| .sp |
| The first part matches an optional opening parenthesis, and if that |
| character is present, sets it as the first captured substring. The second 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. Otherwise, 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. |
| .P |
| If you were embedding this pattern in a larger one, you could use a relative |
| reference: |
| .sp |
| ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ... |
| .sp |
| This makes the fragment independent of the parentheses in the larger pattern. |
| . |
| .SS "Checking for a used subpattern by name" |
| .rs |
| .sp |
| 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 syntax, 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 number, which must be greater than zero. Using subpattern |
| names that consist entirely of digits is not recommended. |
| .P |
| Rewriting the above example to use a named subpattern gives this: |
| .sp |
| (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) ) |
| .sp |
| 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. |
| . |
| .SS "Checking for pattern recursion" |
| .rs |
| .sp |
| 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 ampersand follow the |
| letter R, for example: |
| .sp |
| (?(R3)...) or (?(R&name)...) |
| .sp |
| 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. |
| .P |
| At "top level", all these recursion test conditions are false. |
| .\" HTML <a href="#recursion"> |
| .\" </a> |
| The syntax for recursive patterns |
| .\" |
| is described below. |
| . |
| .\" HTML <a name="subdefine"></a> |
| .SS "Defining subpatterns for use by reference only" |
| .rs |
| .sp |
| 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 referenced from elsewhere. (The use of |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </a> |
| 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): |
| .sp |
| (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) ) |
| \eb (?&byte) (\e.(?&byte)){3} \eb |
| .sp |
| 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, insisting on a word boundary at each end. |
| . |
| .SS "Assertion conditions" |
| .rs |
| .sp |
| 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: |
| .sp |
| (?(?=[^a-z]*[a-z]) |
| \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} ) |
| .sp |
| 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. |
| . |
| . |
| .\" HTML <a name="comments"></a> |
| .SH COMMENTS |
| .rs |
| .sp |
| 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 character class, |
| nor in the middle of any other sequence of related characters such as (?: or a |
| subpattern name or number. The characters that make up a comment play no part |
| in the pattern matching. |
| .P |
| 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 characters are interpreted as newlines |
| is controlled by the options passed to \fBpcre_compile()\fP or by a special |
| sequence at the start of the pattern, as described in the section entitled |
| .\" HTML <a href="#newlines"> |
| .\" </a> |
| "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: |
| .sp |
| abc #comment \en still comment |
| .sp |
| On encountering the # character, \fBpcre_compile()\fP skips along, looking for |
| a newline in the pattern. The sequence \en 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. |
| . |
| . |
| .\" HTML <a name="recursion"></a> |
| .SH "RECURSIVE PATTERNS" |
| .rs |
| .sp |
| 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. |
| .P |
| For some time, Perl has provided a facility that allows regular expressions 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: |
| .sp |
| $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x; |
| .sp |
| The (?p{...}) item interpolates Perl code at run time, and in this case refers |
| recursively to the pattern in which it appears. |
| .P |
| 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. |
| .P |
| 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 |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </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. |
| .P |
| This PCRE pattern solves the nested parentheses problem (assume the |
| PCRE_EXTENDED option is set so that white space is ignored): |
| .sp |
| \e( ( [^()]++ | (?R) )* \e) |
| .sp |
| 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 parenthesized substring). |
| Finally there is a closing parenthesis. Note the use of a possessive quantifier |
| to avoid backtracking into sequences of non-parentheses. |
| .P |
| If this were part of a larger pattern, you would not want to recurse the entire |
| pattern, so instead you could use this: |
| .sp |
| ( \e( ( [^()]++ | (?1) )* \e) ) |
| .sp |
| We have put the pattern into parentheses, and caused the recursion to refer to |
| them instead of the whole pattern. |
| .P |
| 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. |
| .P |
| 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 referenced. They are always |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </a> |
| non-recursive subroutine |
| .\" |
| calls, as described in the next section. |
| .P |
| 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: |
| .sp |
| (?<pn> \e( ( [^()]++ | (?&pn) )* \e) ) |
| .sp |
| If there is more than one subpattern with the same name, the earliest one is |
| used. |
| .P |
| 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 pattern to strings |
| that do not match. For example, when this pattern is applied to |
| .sp |
| (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() |
| .sp |
| 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. |
| .P |
| 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 |
| .\" HREF |
| \fBpcrecallout\fP |
| .\" |
| documentation). If the pattern above is matched against |
| .sp |
| (ab(cd)ef) |
| .sp |
| 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 subpattern 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. |
| .P |
| 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 |
| \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can |
| be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. |
| .P |
| Do not confuse the (?R) item with the condition (R), which tests for recursion. |
| Consider this pattern, which matches text in angle brackets, allowing for |
| arbitrary nesting. Only digits are allowed in nested brackets (that is, when |
| recursing), whereas any characters are permitted at the outer level. |
| .sp |
| < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * > |
| .sp |
| 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. |
| . |
| . |
| .\" HTML <a name="recursiondifference"></a> |
| .SS "Differences in recursion processing between PCRE and Perl" |
| .rs |
| .sp |
| 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 palindromic string that contains an odd number of |
| characters (for example, "a", "aba", "abcba", "abcdcba"): |
| .sp |
| ^(.|(.)(?1)\e2)$ |
| .sp |
| 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": |
| .P |
| 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 alternative is taken |
| and the recursion kicks in. The recursive call to subpattern 1 successfully |
| matches the next character ("b"). (Note that the beginning and end of line |
| tests are not part of the recursion). |
| .P |
| 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: |
| .sp |
| ^((.)(?1)\e2|.)$ |
| .sp |
| 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. |
| .P |
| 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: |
| .sp |
| ^((.)(?1)\e2|.?)$ |
| .sp |
| 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 alternatives at the higher level: |
| .sp |
| ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.)) |
| .sp |
| If you want to match typical palindromic phrases, the pattern has to ignore all |
| non-word characters, which can be done like this: |
| .sp |
| ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$ |
| .sp |
| 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 backtracking 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. |
| .P |
| \fBWARNING\fP: The palindrome-matching patterns above work only if the subject |
| 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 alternatives, so the entire match fails. |
| .P |
| The second way in which PCRE and Perl differ in their recursion processing is |
| in the handling of captured values. In Perl, when a subpattern is called |
| recursively or as a subpattern (see the next section), it has no access to any |
| values that were captured outside the recursion, whereas in PCRE these values |
| can be referenced. Consider this pattern: |
| .sp |
| ^(.)(\e1|a(?2)) |
| .sp |
| In PCRE, this pattern matches "bab". The first capturing parentheses match "b", |
| then in the second group, when the back reference \e1 fails to match "b", the |
| second alternative matches "a" and then recurses. In the recursion, \e1 does |
| now match "b" and so the whole match succeeds. In Perl, the pattern fails to |
| match because inside the recursive call \e1 cannot access the externally set |
| value. |
| . |
| . |
| .\" HTML <a name="subpatternsassubroutines"></a> |
| .SH "SUBPATTERNS AS SUBROUTINES" |
| .rs |
| .sp |
| 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: |
| .sp |
| (...(absolute)...)...(?2)... |
| (...(relative)...)...(?-1)... |
| (...(?+1)...(relative)... |
| .sp |
| An earlier example pointed out that the pattern |
| .sp |
| (sens|respons)e and \e1ibility |
| .sp |
| matches "sense and sensibility" and "response and responsibility", but not |
| "sense and responsibility". If instead the pattern |
| .sp |
| (sens|respons)e and (?1)ibility |
| .sp |
| 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. |
| .P |
| All subroutine calls, whether recursive or not, are always treated as atomic |
| groups. That is, once a subroutine 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. Any capturing parentheses that are set during the |
| subroutine call revert to their previous values afterwards. |
| .P |
| Processing options such as case-independence are fixed when a subpattern is |
| defined, so if it is used as a subroutine, such options cannot be changed for |
| different calls. For example, consider this pattern: |
| .sp |
| (abc)(?i:(?-1)) |
| .sp |
| It matches "abcabc". It does not match "abcABC" because the change of |
| processing option does not affect the called subpattern. |
| . |
| . |
| .\" HTML <a name="onigurumasubroutines"></a> |
| .SH "ONIGURUMA SUBROUTINE SYNTAX" |
| .rs |
| .sp |
| For compatibility with Oniguruma, the non-Perl syntax \eg 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, rewritten using this syntax: |
| .sp |
| (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) ) |
| (sens|respons)e and \eg'1'ibility |
| .sp |
| 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: |
| .sp |
| (abc)(?i:\eg<-1>) |
| .sp |
| Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP |
| synonymous. The former is a back reference; the latter is a subroutine call. |
| . |
| . |
| .SH CALLOUTS |
| .rs |
| .sp |
| 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 substrings that match the |
| same pair of parentheses when there is a repetition. |
| .P |
| 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 \fIpcre_callout\fP. |
| By default, this variable contains NULL, which disables all calling out. |
| .P |
| 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: |
| .sp |
| (?C1)abc(?C2)def |
| .sp |
| If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are |
| automatically installed before each item in the pattern. They are all numbered |
| 255. |
| .P |
| During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP 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 \fBpcre_exec()\fP. The callout function |
| may cause matching to proceed, to backtrack, or to fail altogether. A complete |
| description of the interface to the callout function is given in the |
| .\" HREF |
| \fBpcrecallout\fP |
| .\" |
| documentation. |
| . |
| . |
| .\" HTML <a name="backtrackcontrol"></a> |
| .SH "BACKTRACKING CONTROL" |
| .rs |
| .sp |
| Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which |
| are described in the Perl documentation as "experimental and subject 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. |
| .P |
| Since these verbs are specifically related to backtracking, most of them can be |
| used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses |
| a backtracking algorithm. With the exception of (*FAIL), which behaves like a |
| failing negative assertion, they cause an error if encountered by |
| \fBpcre_dfa_exec()\fP. |
| .P |
| 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 pattern, with one |
| exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in |
| a successful positive assertion \fIis\fP 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 different in some cases. |
| .P |
| The new verbs make use of what was previously invalid syntax: an opening |
| parenthesis followed by an asterisk. They are generally of the form |
| (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour, |
| 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. |
| .P |
| 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 \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the |
| pattern with (*NO_START_OPT). |
| .P |
| Experiments with Perl suggest that it too has similar optimizations, sometimes |
| leading to anomalous results. |
| . |
| . |
| .SS "Verbs that act immediately" |
| .rs |
| .sp |
| The following verbs act as soon as they are encountered. They may not be |
| followed by a name. |
| .sp |
| (*ACCEPT) |
| .sp |
| 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: |
| .sp |
| A((?:A|B(*ACCEPT)|C)D) |
| .sp |
| This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by |
| the outer parentheses. |
| .sp |
| (*FAIL) or (*F) |
| .sp |
| 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 pattern: |
| .sp |
| a+(?C)(*FAIL) |
| .sp |
| A match with the string "aaaa" always fails, but the callout is taken before |
| each backtrack happens (in this example, 10 times). |
| . |
| . |
| .SS "Recording which path was taken" |
| .rs |
| .sp |
| 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). |
| .sp |
| (*MARK:NAME) or (*:NAME) |
| .sp |
| 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. |
| .P |
| When a match succeeds, the name of the last-encountered (*MARK) on the matching |
| path is passed back to the caller via the \fIpcre_extra\fP data structure, as |
| described in the |
| .\" HTML <a href="pcreapi.html#extradata"> |
| .\" </a> |
| section on \fIpcre_extra\fP |
| .\" |
| in the |
| .\" HREF |
| \fBpcreapi\fP |
| .\" |
| documentation. Here is an example of \fBpcretest\fP output, where the /K |
| modifier requests the retrieval and outputting of (*MARK) data: |
| .sp |
| re> /X(*MARK:A)Y|X(*MARK:B)Z/K |
| data> XY |
| 0: XY |
| MK: A |
| XZ |
| 0: XZ |
| MK: B |
| .sp |
| The (*MARK) name is tagged with "MK:" in this output, and in this example it |
| indicates which of the two alternatives matched. This is a more efficient way |
| of obtaining this information than putting each alternative in its own |
| capturing parentheses. |
| .P |
| 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. |
| .P |
| After a partial match or a failed match, the name of the last encountered |
| (*MARK) in the entire match process is returned. For example: |
| .sp |
| re> /X(*MARK:A)Y|X(*MARK:B)Z/K |
| data> XP |
| No match, mark = B |
| .sp |
| 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. |
| . |
| . |
| .SS "Verbs that act after backtracking" |
| .rs |
| .sp |
| The following verbs do nothing when they are encountered. Matching continues |
| 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 backtracking 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.) |
| .P |
| These verbs differ in exactly what kind of failure occurs when backtracking |
| reaches them. |
| .sp |
| (*COMMIT) |
| .sp |
| 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, \fBpcre_exec()\fP is committed to |
| finding a match at the current starting point, or not at all. For example: |
| .sp |
| a+(*COMMIT)b |
| .sp |
| 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. |
| .P |
| 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 |
| \fBpcretest\fP example: |
| .sp |
| re> /(*COMMIT)abc/ |
| data> xyzabc |
| 0: abc |
| xyzabc\eY |
| No match |
| .sp |
| 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 \eY escape in the second subject, the match |
| starts at "x" and so the (*COMMIT) causes it to fail without trying any other |
| starting points. |
| .sp |
| (*PRUNE) or (*PRUNE:NAME) |
| .sp |
| 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 alternative to an atomic group or possessive quantifier, |
| but there are some uses of (*PRUNE) that cannot be expressed in any other way. |
| The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an |
| anchored pattern (*PRUNE) has the same effect as (*COMMIT). |
| .sp |
| (*SKIP) |
| .sp |
| 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 encountered. (*SKIP) |
| signifies that whatever text was matched leading up to it cannot be part of a |
| successful match. Consider: |
| .sp |
| a+(*SKIP)b |
| .sp |
| 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 quantifer 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". |
| .sp |
| (*SKIP:NAME) |
| .sp |
| 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 corresponds to that |
| (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a |
| matching name is found, the (*SKIP) is ignored. |
| .sp |
| (*THEN) or (*THEN:NAME) |
| .sp |
| This verb causes a skip to the next innermost alternative if the rest of the |
| pattern does not match. That is, it cancels pending backtracking, 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: |
| .sp |
| ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... |
| .sp |
| 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). |
| .P |
| 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 subpattern to the |
| enclosing alternative. Consider this pattern, where A, B, etc. are complex |
| pattern fragments that do not contain any | characters at this level: |
| .sp |
| A (B(*THEN)C) | D |
| .sp |
| 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: |
| .sp |
| A (B(*THEN)C | (*FAIL)) | D |
| .sp |
| The effect of (*THEN) is now confined to the inner subpattern. After a failure |
| in C, matching moves to (*FAIL), which causes the whole subpattern to fail |
| because there are no more alternatives to try. In this case, matching does now |
| backtrack into A. |
| .P |
| 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: |
| .sp |
| ^.*? (?(?=a) a | b(*THEN)c ) |
| .sp |
| 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.) |
| .P |
| 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. |
| .P |
| 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: |
| .sp |
| (A(*COMMIT)B(*THEN)C|D) |
| .sp |
| Once A has matched, PCRE is committed to this match, at the current starting |
| position. If subsequently B matches, but C does not, the normal (*THEN) action |
| of trying the next alternative (that is, D) does not happen because (*COMMIT) |
| overrides. |
| . |
| . |
| .SH "SEE ALSO" |
| .rs |
| .sp |
| \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), |
| \fBpcresyntax\fP(3), \fBpcre\fP(3). |
| . |
| . |
| .SH AUTHOR |
| .rs |
| .sp |
| .nf |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| .fi |
| . |
| . |
| .SH REVISION |
| .rs |
| .sp |
| .nf |
| Last updated: 29 November 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| .fi |