NAME
PCRE - Perl-compatible regular expressions
DIFFERENCES BETWEEN PCRE AND 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 5.10.
1. PCRE has only a subset of Perl’s UTF-8 and Unicode support. Details
of what it does have are given in the 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 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).
12. 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.
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.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 04 October 2009
Copyright (c) 1997-2009 University of Cambridge.