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 5.8.

1. PCRE does not have full UTF-8 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.

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 (?p{code}) constructions. However, there is support for recursive patterns using the non-Perl items (?R), (?number), and (?P>name). Also, the PCRE "callout" feature allows an external function to be called during pattern matching. See the pcrecallout documentation for details.

9. 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".

10. PCRE provides some extensions to the Perl regular expression facilities: (a) Although lookbehind assertions 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. (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, and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equivalents. (g) The (?R), (?number), and (?P>name) constructs allows for recursive pattern matching (Perl can do this using the (?p{code}) construct, which PCRE cannot support.) (h) PCRE supports named capturing substrings, using the Python syntax. (i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java package. (j) The (R) condition, for testing recursion, is a PCRE extension. (k) The callout facility is PCRE-specific. (l) The partial matching facility is PCRE-specific. (m) Patterns compiled by PCRE can be saved and re-used at a later time, even on different hosts that have the other endianness. (n) The alternative matching function (pcre_dfa_exec()) matches in a different way and is not Perl-compatible.

Last updated: 28 February 2005

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