Provided by: libpcre3-dev_8.31-2ubuntu2.3_amd64 
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 versions 5.10 and above.
1. PCRE has only a subset of Perl's Unicode support. Details of what it does have are given in the
pcreunicode page.
2. PCRE allows repeat quantifiers only on parenthesized assertions, 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 (in principle: PCRE optimizes this to run the assertion
just once). Perl allows repeat quantifiers on other assertions such as \b, but these do not seem to have
any use.
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 when followed by a
character name or Unicode value. (\N on its own, matching a non-newline character, is supported.) 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 by default. However, if the
PCRE_JAVASCRIPT_COMPAT option is set, \U and \u are interpreted as JavaScript interprets them.
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 implements a simpler version of \X than Perl, which changed to make \X match what Unicode calls
an "extended grapheme cluster". This is more complicated than an extended Unicode sequence, which is what
PCRE matches.
8. 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$xyz
The \Q...\E sequence is recognized both inside and outside character classes.
9. Fairly obviously, PCRE does not support the (?{code}) and (??{code}) constructions. However, there is
support for recursive patterns. This is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE
"callout" feature allows an external function to be called during pattern matching. See the pcrecallout
documentation for details.
10. Subpatterns that are called as subroutines (whether or not recursively) are always treated as atomic
groups in PCRE. This is like Python, but unlike Perl. Captured values that are set outside a subroutine
call can be reference from inside in PCRE, but not in Perl. There is a discussion that explains these
differences in more detail in the section on recursion differences from Perl in the pcrepattern page.
11. If any of the backtracking control verbs are used in an assertion or in a subpattern that is called
as a subroutine (whether or not recursively), their effect is confined to that subpattern; it does not
extend to the surrounding pattern. This is not always the case in Perl. In particular, if (*THEN) is
present in a group that is called as a subroutine, its action is limited to that group, even if the group
does not contain any | characters. There is one exception to this: the name from a *(MARK), (*PRUNE), or
(*THEN) that is encountered in a successful positive assertion is passed back when a match succeeds
(compare capturing parentheses in assertions). Note that such subpatterns are processed as anchored at
the point where they are tested.
12. There are some differences that are concerned with the settings of captured strings when part of a
pattern is repeated. For example, matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
unset, but in PCRE it is set to "b".
13. 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.
14. Perl recognizes comments in some places that PCRE does not, for example, between the ( and ? at the
start of a subpattern. If the /x modifier is set, Perl allows white space between ( and ? but PCRE never
does, even if the PCRE_EXTENDED option is set.
15. 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. However, this does not apply to optimized data created by the just-in-time
compiler.
(k) The alternative matching functions (pcre_dfa_exec() and pcre16_dfa_exec()) match in a different way
and are 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: 01 June 2012
Copyright (c) 1997-2012 University of Cambridge.