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NAME | DIFFERENCES BETWEEN PCRE AND PERL | AUTHOR | REVISION | COLOPHON |
PCRECOMPAT(3) Library Functions Manual PCRECOMPAT(3)
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
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 sometimes (but not always) sets its numerical
variables from inside negative assertions.
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 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$xyz
The \Q...\E sequence is recognized both inside and outside character
classes.
8. Fairly obviously, PCRE does not support the (?{code}) and
(??{code}) constructions. However, there is support for recursive
patterns. This is not available in Perl 5.8, but it is in Perl 5.10.
Also, the PCRE "callout" feature allows an external function to be
called during pattern matching. See the pcrecallout documentation for
details.
9. Subpatterns that are called as subroutines (whether or not
recursively) are always treated as atomic groups in PCRE. This is
like Python, but unlike Perl. Captured values that are set outside a
subroutine call can be reference from inside in PCRE, but not in
Perl. There is a discussion that explains these differences in more
detail in the section on recursion differences from Perl in the
pcrepattern page.
10. If any of the backtracking control verbs are used in a subpattern
that is called as a subroutine (whether or not recursively), their
effect is confined to that subpattern; it does not extend to the
surrounding pattern. This is not always the case in Perl. In
particular, if (*THEN) is present in a group that is called as a
subroutine, its action is limited to that group, even if the group
does not contain any | characters. Note that such subpatterns are
processed as anchored at the point where they are tested.
11. If a pattern contains more than one backtracking control verb,
the first one that is backtracked onto acts. For example, in the
pattern A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a
failure in C triggers (*PRUNE). Perl's behaviour is more complex; in
many cases it is the same as PCRE, but there are examples where it
differs.
12. Most backtracking verbs in assertions have their normal actions.
They are not confined to the assertion.
13. There are some differences that are concerned with the settings
of captured strings when part of a pattern is repeated. For example,
matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
unset, but in PCRE it is set to "b".
14. 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.
15. 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 ? (though
current Perls warn that this is deprecated) but PCRE never does, even
if the PCRE_EXTENDED option is set.
16. Perl, when in warning mode, gives warnings for character classes
such as [A-\d] or [a-[:digit:]]. It then treats the hyphens as
literals. PCRE has no warning features, so it gives an error in these
cases because they are almost certainly user mistakes.
17. In PCRE, the upper/lower case character properties Lu and Ll are
not affected when case-independent matching is specified. For
example, \p{Lu} always matches an upper case letter. I think Perl has
changed in this respect; in the release at the time of writing
(5.16), \p{Lu} and \p{Ll} match all letters, regardless of case, when
case independence is specified.
18. 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(),
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.
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
Last updated: 10 November 2013
Copyright (c) 1997-2013 University of Cambridge.
This page is part of the PCRE (Perl Compatible Regular Expressions)
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PCRE 8.34 10 November 2013 PCRECOMPAT(3)