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Section: C Library Functions (3)
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PCRE - Perl-compatible regular expressions  


This document describes the differences in the ways that PCRE and Perl handle regular expressions. The differences described here are with respect to Perl versions 5.10 and above.

1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what it does have are given in the section on UTF-8 support in the main pcre page.

2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits them, 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.

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.

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.

5. The following Perl escape sequences are not supported: \l, \u, \L, \U, and \N. 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.

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."

7. 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:

    Pattern            PCRE matches      Perl matches

    \Qabc$xyz\E        abc$xyz           abc followed by the
                                           contents of $xyz
    \Qabc\$xyz\E       abc\$xyz          abc\$xyz
    \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz

The \Q...\E sequence is recognized both inside and outside character classes.

8. 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 pcrecallout documentation for details.

9. Subpatterns that are called recursively or as "subroutines" are always treated as atomic groups in PCRE. This is like Python, but unlike Perl. There is a discussion of an example that explains this in more detail in the section on recursion differences from Perl in the pcrepattern page.

10. 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".

11. 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.

12. Perl recognizes comments in some places that PCRE doesn't, for example, between the ( and ? at the start of a subpattern.

13. 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:

(a) Although lookbehind assertions in PCRE must match fixed length strings, each alternative branch of a lookbehind assertion can match a different length of string. Perl requires them all to have the same length.

(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta-character matches only at the very end of the string.

(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored. (Perl can be made to issue a warning.)

(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is inverted, that is, by default they are not greedy, but if followed by a question mark they are.

(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried only at the first matching position in the subject string.

(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equivalents.

(g) The \R escape sequence can be restricted to match only CR, LF, or CRLF by the PCRE_BSR_ANYCRLF option.

(h) The callout facility is PCRE-specific.

(i) The partial matching facility is PCRE-specific.

(j) Patterns compiled by PCRE can be saved and re-used at a later time, even on different hosts that have the other endianness.

(k) The alternative matching function (pcre_dfa_exec()) matches in a different way and is not Perl-compatible.

(l) PCRE recognizes some special sequences such as (*CR) at the start of a pattern that set overall options that cannot be changed within the pattern.  


Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.


Last updated: 31 October 2010
Copyright (c) 1997-2010 University of Cambridge.




This document was created by man2html, using the manual pages.
Time: 21:52:09 GMT, April 16, 2011