PCRECOMPAT(3)PCRECOMPAT(3)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 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, 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_COM‐
PAT 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 prop‐
erties 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 representa‐
tion 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. 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$xyz
The \Q...\E sequence is recognized both inside and outside character
classes.
8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
constructions. However, there is support for recursive patterns. This
is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE
"callout" feature allows an external function to be called during pat‐
tern matching. See the pcrecallout documentation for details.
9. Subpatterns that are called as subroutines (whether or not recur‐
sively) are always treated as atomic groups in PCRE. This is like
Python, but unlike Perl. Captured values that are set outside a sub‐
routine call can be reference from inside in PCRE, but not in Perl.
There is a discussion that explains these differences in more detail in
the section on recursion differences from Perl in the pcrepattern page.
10. If any of the backtracking control verbs are used in an assertion
or in a subpattern that is called as a subroutine (whether or not
recursively), their effect is confined to that subpattern; it does not
extend to the surrounding pattern. This is not always the case in Perl.
In particular, if (*THEN) is present in a group that is called as a
subroutine, its action is limited to that group, even if the group does
not contain any | characters. There is one exception to this: the name
from a *(MARK), (*PRUNE), or (*THEN) that is encountered in a success‐
ful positive assertion is passed back when a match succeeds (compare
capturing parentheses in assertions). Note that such subpatterns are
processed as anchored at the point where they are tested.
11. There are some differences that are concerned with the settings of
captured strings when part of a pattern is repeated. For example,
matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
unset, but in PCRE it is set to "b".
12. PCRE's handling of duplicate subpattern numbers and duplicate sub‐
pattern 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 ta‐
ble 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 cap‐
turing subpattern number 1. To avoid this confusing situation, an error
is given at compile time.
13. 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.
14. PCRE provides some extensions to the Perl regular expression facil‐
ities. Perl 5.10 includes new features that are not in earlier ver‐
sions 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 spe‐
cial 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 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 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 equiva‐
lents.
(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(),
pcre16_dfa_exec() and pcre32_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: 25 August 2012
Copyright (c) 1997-2012 University of Cambridge.
PCRE 8.30 24 June 2012 PCRECOMPAT(3)
