PCRE(3)PCRE(3)NAME
PCRE - Perl-compatible regular expressions
DIFFERENCES FROM PERL
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 asser‐
tions 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 succeed‐
ing), 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 nor‐
mal 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, \P, \p, \N, and \X. 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. PCRE does support the \Q...\E escape for quoting substrings. Charac‐
ters 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$xyz
The \Q...\E sequence is recognized both inside and outside character
classes.
7. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
constructions. However, there is some experimental support for recur‐
sive patterns using the non-Perl items (?R), (?number) and (?P>name).
Also, the PCRE "callout" feature allows an external function to be
called during pattern matching.
8. There are some differences that are concerned with the settings of
captured strings when part of a pattern is repeated. For example,
matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
unset, but in PCRE it is set to "b".
9. PCRE provides some extensions to the Perl regular expression facili‐
ties:
(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 spe‐
cial meaning is faulted.
(d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti‐
fiers is inverted, that is, by default they are not greedy, but if fol‐
lowed by a question mark they are.
(e) PCRE_ANCHORED can be used 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_CAP‐
TURE 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.
Last updated: 09 December 2003Copyright (c) 1997-2003 University of Cambridge.
PCRE(3)