| <html> |
| <head> |
| <title>pcrecompat specification</title> |
| </head> |
| <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB"> |
| <h1>pcrecompat man page</h1> |
| <p> |
| Return to the <a href="index.html">PCRE index page</a>. |
| </p> |
| <p> |
| This page is part of the PCRE HTML documentation. It was generated automatically |
| from the original man page. If there is any nonsense in it, please consult the |
| man page, in case the conversion went wrong. |
| <br> |
| <br><b> |
| DIFFERENCES BETWEEN PCRE AND PERL |
| </b><br> |
| <P> |
| This document describes the differences in the ways that PCRE and Perl handle |
| regular expressions. The differences described here are with respect to Perl |
| versions 5.10 and above. |
| </P> |
| <P> |
| 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what |
| it does have are given in the |
| <a href="pcreunicode.html"><b>pcreunicode</b></a> |
| page. |
| </P> |
| <P> |
| 2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do |
| not mean what you might think. For example, (?!a){3} does not assert that the |
| next three characters are not "a". It just asserts that the next character is |
| not "a" three times (in principle: PCRE optimizes this to run the assertion |
| just once). Perl allows repeat quantifiers on other assertions such as \b, but |
| these do not seem to have any use. |
| </P> |
| <P> |
| 3. Capturing subpatterns that occur inside negative lookahead assertions are |
| counted, but their entries in the offsets vector are never set. Perl sets its |
| numerical variables from any such patterns that are matched before the |
| assertion fails to match something (thereby succeeding), but only if the |
| negative lookahead assertion contains just one branch. |
| </P> |
| <P> |
| 4. Though binary zero characters are supported in the subject string, they are |
| not allowed in a pattern string because it is passed as a normal C string, |
| terminated by zero. The escape sequence \0 can be used in the pattern to |
| represent a binary zero. |
| </P> |
| <P> |
| 5. The following Perl escape sequences are not supported: \l, \u, \L, |
| \U, and \N when followed by a character name or Unicode value. (\N on its |
| own, matching a non-newline character, is supported.) In fact these are |
| implemented by Perl's general string-handling and are not part of its pattern |
| matching engine. If any of these are encountered by PCRE, an error is |
| generated by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set, |
| \U and \u are interpreted as JavaScript interprets them. |
| </P> |
| <P> |
| 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is |
| built with Unicode character property support. The properties that can be |
| tested with \p and \P are limited to the general category properties such as |
| Lu and Nd, script names such as Greek or Han, and the derived properties Any |
| and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the |
| Perl documentation says "Because Perl hides the need for the user to understand |
| the internal representation of Unicode characters, there is no need to |
| implement the somewhat messy concept of surrogates." |
| </P> |
| <P> |
| 7. PCRE implements a simpler version of \X than Perl, which changed to make |
| \X match what Unicode calls an "extended grapheme cluster". This is more |
| complicated than an extended Unicode sequence, which is what PCRE matches. |
| </P> |
| <P> |
| 8. PCRE does support the \Q...\E escape for quoting substrings. Characters in |
| between are treated as literals. This is slightly different from Perl in that $ |
| and @ are also handled as literals inside the quotes. In Perl, they cause |
| variable interpolation (but of course PCRE does not have variables). Note the |
| following examples: |
| <pre> |
| Pattern PCRE matches Perl matches |
| |
| \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz |
| \Qabc\$xyz\E abc\$xyz abc\$xyz |
| \Qabc\E\$\Qxyz\E abc$xyz abc$xyz |
| </pre> |
| The \Q...\E sequence is recognized both inside and outside character classes. |
| </P> |
| <P> |
| 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code}) |
| constructions. However, there is support for recursive patterns. This is not |
| available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout" |
| feature allows an external function to be called during pattern matching. See |
| the |
| <a href="pcrecallout.html"><b>pcrecallout</b></a> |
| documentation for details. |
| </P> |
| <P> |
| 10. Subpatterns that are called as subroutines (whether or not recursively) are |
| always treated as atomic groups in PCRE. This is like Python, but unlike Perl. |
| Captured values that are set outside a subroutine call can be reference from |
| inside in PCRE, but not in Perl. There is a discussion that explains these |
| differences in more detail in the |
| <a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a> |
| in the |
| <a href="pcrepattern.html"><b>pcrepattern</b></a> |
| page. |
| </P> |
| <P> |
| 11. If (*THEN) is present in a group that is called as a subroutine, its action |
| is limited to that group, even if the group does not contain any | characters. |
| </P> |
| <P> |
| 12. There are some differences that are concerned with the settings of captured |
| strings when part of a pattern is repeated. For example, matching "aba" against |
| the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b". |
| </P> |
| <P> |
| 13. PCRE's handling of duplicate subpattern numbers and duplicate subpattern |
| names is not as general as Perl's. This is a consequence of the fact the PCRE |
| works internally just with numbers, using an external table to translate |
| between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B), |
| where the two capturing parentheses have the same number but different names, |
| is not supported, and causes an error at compile time. If it were allowed, it |
| would not be possible to distinguish which parentheses matched, because both |
| names map to capturing subpattern number 1. To avoid this confusing situation, |
| an error is given at compile time. |
| </P> |
| <P> |
| 14. Perl recognizes comments in some places that PCRE does not, for example, |
| between the ( and ? at the start of a subpattern. If the /x modifier is set, |
| Perl allows whitespace between ( and ? but PCRE never does, even if the |
| PCRE_EXTENDED option is set. |
| </P> |
| <P> |
| 15. PCRE provides some extensions to the Perl regular expression facilities. |
| Perl 5.10 includes new features that are not in earlier versions of Perl, some |
| of which (such as named parentheses) have been in PCRE for some time. This list |
| is with respect to Perl 5.10: |
| <br> |
| <br> |
| (a) Although lookbehind assertions in PCRE must match fixed length strings, |
| each alternative branch of a lookbehind assertion can match a different length |
| of string. Perl requires them all to have the same length. |
| <br> |
| <br> |
| (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ |
| meta-character matches only at the very end of the string. |
| <br> |
| <br> |
| (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special |
| meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored. |
| (Perl can be made to issue a warning.) |
| <br> |
| <br> |
| (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is |
| inverted, that is, by default they are not greedy, but if followed by a |
| question mark they are. |
| <br> |
| <br> |
| (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried |
| only at the first matching position in the subject string. |
| <br> |
| <br> |
| (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and |
| PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents. |
| <br> |
| <br> |
| (g) The \R escape sequence can be restricted to match only CR, LF, or CRLF |
| by the PCRE_BSR_ANYCRLF option. |
| <br> |
| <br> |
| (h) The callout facility is PCRE-specific. |
| <br> |
| <br> |
| (i) The partial matching facility is PCRE-specific. |
| <br> |
| <br> |
| (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on |
| different hosts that have the other endianness. However, this does not apply to |
| optimized data created by the just-in-time compiler. |
| <br> |
| <br> |
| (k) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a |
| different way and is not Perl-compatible. |
| <br> |
| <br> |
| (l) PCRE recognizes some special sequences such as (*CR) at the start of |
| a pattern that set overall options that cannot be changed within the pattern. |
| </P> |
| <br><b> |
| AUTHOR |
| </b><br> |
| <P> |
| Philip Hazel |
| <br> |
| University Computing Service |
| <br> |
| Cambridge CB2 3QH, England. |
| <br> |
| </P> |
| <br><b> |
| REVISION |
| </b><br> |
| <P> |
| Last updated: 14 November 2011 |
| <br> |
| Copyright © 1997-2011 University of Cambridge. |
| <br> |
| <p> |
| Return to the <a href="index.html">PCRE index page</a>. |
| </p> |