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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 mainly with respect to
Perl 5.8, though PCRE versions 7.0 and later contain some features that are
expected to be in the forthcoming Perl 5.10.
1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
it does have are given in the
.\" HTML <a href="pcre.html#utf8support">
.\" </a>
section on UTF-8 support
in the main
.\" HREF
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 \e0 can be used in the pattern to
represent a binary zero.
5. The following Perl escape sequences are not supported: \el, \eu, \eL,
\eU, and \eN. 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 \ep, \eP, and \eX are supported only if PCRE is
built with Unicode character property support. The properties that can be
tested with \ep and \eP 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&.
7. PCRE does support the \eQ...\eE 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
.\" 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$xyz
The \eQ...\eE sequence is recognized both inside and outside character classes.
8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
constructions. However, there is support for recursive patterns. This is not
available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"
feature allows an external function to be called during pattern matching. See
.\" HREF
documentation for details.
9. Subpatterns that are called recursively or as "subroutines" are always
treated as atomic groups in PCRE. This is like Python, but unlike Perl.
10. There are some differences that are concerned with the settings of captured
strings when part of a pattern is repeated. For example, matching "aba" against
the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),
(*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an
argument. PCRE does not support (*MARK). If (*ACCEPT) is within capturing
parentheses, PCRE does not set that capture group; this is different to Perl.
12. PCRE provides some extensions to the Perl regular expression facilities.
Perl 5.10 will include new features that are not in earlier versions, 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 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.
options for \fBpcre_exec()\fP have no Perl equivalents.
(g) The callout facility is PCRE-specific.
(h) The partial matching facility is PCRE-specific.
(i) Patterns compiled by PCRE can be saved and re-used at a later time, even on
different hosts that have the other endianness.
(j) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
different way and is not Perl-compatible.
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
Last updated: 08 August 2007
Copyright (c) 1997-2007 University of Cambridge.