pcre2compat
PCRE2COMPAT(3) Library Functions Manual PCRE2COMPAT(3)
NAME
PCRE2 - Perl-compatible regular expressions (revised API)
DIFFERENCES BETWEEN PCRE2 AND PERL
This document describes some of the differences in the ways that PCRE2
and Perl handle regular expressions. The differences described here are
with respect to Perl version 5.34.0, but as both Perl and PCRE2 are
continually changing, the information may at times be out of date.
1. When PCRE2_DOTALL (equivalent to Perl's /s qualifier) is not set,
the behaviour of the '.' metacharacter differs from Perl. In PCRE2, '.'
matches the next character unless it is the start of a newline se-
quence. This means that, if the newline setting is CR, CRLF, or NUL,
'.' will match the code point LF (0x0A) in ASCII/Unicode environments,
and NL (either 0x15 or 0x25) when using EBCDIC. In Perl, '.' appears
never to match LF, even when 0x0A is not a newline indicator.
2. PCRE2 has only a subset of Perl's Unicode support. Details of what
it does have are given in the pcre2unicode page.
3. Like Perl, PCRE2 allows repeat quantifiers on parenthesized asser-
tions, 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 as-
serts that the next character is not "a" three times (in principle;
PCRE2 optimizes this to run the assertion just once). Perl allows some
repeat quantifiers on other assertions, for example, \b* , but these do
not seem to have any use. PCRE2 does not allow any kind of quantifier
on non-lookaround assertions.
4. Capture groups that occur inside negative lookaround assertions are
counted, but their entries in the offsets vector are set only when a
negative assertion is a condition that has a matching branch (that is,
the condition is false). Perl may set such capture groups in other
circumstances.
5. The following Perl escape sequences are not supported: \F, \l, \L,
\u, \U, and \N when followed by a character name. \N on its own, match-
ing a non-newline character, and \N{U+dd..}, matching a Unicode code
point, are supported. The escapes that modify the case of following
letters are implemented by Perl's general string-handling and are not
part of its pattern matching engine. If any of these are encountered by
PCRE2, an error is generated by default. However, if either of the
PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX options is set, \U and \u are
interpreted as ECMAScript interprets them.
6. The Perl escape sequences \p, \P, and \X are supported only if PCRE2
is built with Unicode support (the default). 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, Bidi_Class,
Bidi_Control, and the derived properties Any and LC (synonym L&). Both
PCRE2 and Perl support the Cs (surrogate) property, but in PCRE2 its
use is limited. See the pcre2pattern documentation for details. The
long synonyms for property names that Perl supports (such as \p{Let-
ter}) are not supported by PCRE2, nor is it permitted to prefix any of
these properties with "Is".
7. PCRE2 supports the \Q...\E escape for quoting substrings. Characters
in between are treated as literals. However, this is slightly different
from Perl in that $ and @ are also handled as literals inside the
quotes. In Perl, they cause variable interpolation (PCRE2 does not have
variables). Also, Perl does "double-quotish backslash interpolation" on
any backslashes between \Q and \E which, its documentation says, "may
lead to confusing results". PCRE2 treats a backslash between \Q and \E
just like any other character. Note the following examples:
Pattern PCRE2 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$xyz
\QA\B\E A\B A\B
\Q\\E \ \\E
The \Q...\E sequence is recognized both inside and outside character
classes by both PCRE2 and Perl.
8. Fairly obviously, PCRE2 does not support the (?{code}) and
(??{code}) constructions. However, PCRE2 does have a "callout" feature,
which allows an external function to be called during pattern matching.
See the pcre2callout documentation for details.
9. Subroutine calls (whether recursive or not) were treated as atomic
groups up to PCRE2 release 10.23, but from release 10.30 this changed,
and backtracking into subroutine calls is now supported, as in Perl.
10. In PCRE2, if any of the backtracking control verbs are used in a
group that is called as a subroutine (whether or not recursively),
their effect is confined to that group; it does not extend to the sur-
rounding pattern. This is not always the case in Perl. In particular,
if (*THEN) is present in a group that is called as a subroutine, its
action is limited to that group, even if the group does not contain any
| characters. Note that such groups are processed as anchored at the
point where they are tested.
11. If a pattern contains more than one backtracking control verb, the
first one that is backtracked onto acts. For example, in the pattern
A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure
in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases
it is the same as PCRE2, but there are cases where it differs.
12. There are some differences that are concerned with the settings of
captured strings when part of a pattern is repeated. For example,
matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 un-
set, but in PCRE2 it is set to "b".
13. PCRE2's handling of duplicate capture group numbers and names is
not as general as Perl's. This is a consequence of the fact the PCRE2
works internally just with numbers, using an external table to trans-
late between numbers and names. In particular, a pattern such as
(?|(?<a>A)|(?<b>B)), where the two capture groups 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
group matched, because both names map to capture group number 1. To
avoid this confusing situation, an error is given at compile time.
14. Perl used to recognize comments in some places that PCRE2 does not,
for example, between the ( and ? at the start of a group. If the /x
modifier is set, Perl allowed white space between ( and ? though the
latest Perls give an error (for a while it was just deprecated). There
may still be some cases where Perl behaves differently.
15. Perl, when in warning mode, gives warnings for character classes
such as [A-\d] or [a-[:digit:]]. It then treats the hyphens as liter-
als. PCRE2 has no warning features, so it gives an error in these cases
because they are almost certainly user mistakes.
16. In PCRE2, 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 (5.34), \p{Lu} and
\p{Ll} match all letters, regardless of case, when case independence is
specified.
17. From release 5.32.0, Perl locks out the use of \K in lookaround as-
sertions. From release 10.38 PCRE2 does the same by default. However,
there is an option for re-enabling the previous behaviour. When this
option is set, \K is acted on when it occurs in positive assertions,
but is ignored in negative assertions.
18. PCRE2 provides some extensions to the Perl regular expression fa-
cilities. Perl 5.10 included new features that were not in earlier
versions of Perl, some of which (such as named parentheses) were in
PCRE2 for some time before. This list is with respect to Perl 5.34:
(a) Although lookbehind assertions in PCRE2 must match fixed length
strings, each alternative toplevel branch of a lookbehind assertion can
match a different length of string. Perl used to require them all to
have the same length, but the latest version has some variable length
support.
(b) From PCRE2 10.23, backreferences to groups of fixed length are sup-
ported in lookbehinds, provided that there is no possibility of refer-
encing a non-unique number or name. Perl does not support backrefer-
ences in lookbehinds.
(c) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the
$ meta-character matches only at the very end of the string.
(d) A backslash followed by a letter with no special meaning is
faulted. (Perl can be made to issue a warning.)
(e) If PCRE2_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.
(f) PCRE2_ANCHORED can be used at matching time to force a pattern to
be tried only at the first matching position in the subject string.
(g) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and
PCRE2_NOTEMPTY_ATSTART options have no Perl equivalents.
(h) The \R escape sequence can be restricted to match only CR, LF, or
CRLF by the PCRE2_BSR_ANYCRLF option.
(i) The callout facility is PCRE2-specific. Perl supports codeblocks
and variable interpolation, but not general hooks on every match.
(j) The partial matching facility is PCRE2-specific.
(k) The alternative matching function (pcre2_dfa_match() matches in a
different way and is not Perl-compatible.
(l) PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT)
at the start of a pattern. These set overall options that cannot be
changed within the pattern.
(m) PCRE2 supports non-atomic positive lookaround assertions. This is
an extension to the lookaround facilities. The default, Perl-compatible
lookarounds are atomic.
19. The Perl /a modifier restricts /d numbers to pure ascii, and the
/aa modifier restricts /i case-insensitive matching to pure ascii, ig-
noring Unicode rules. This separation cannot be represented with
PCRE2_UCP.
20. Perl has different limits than PCRE2. See the pcre2limit documenta-
tion for details. Perl went with 5.10 from recursion to iteration keep-
ing the intermediate matches on the heap, which is ~10% slower but does
not fall into any stack-overflow limit. PCRE2 made a similar change at
release 10.30, and also has many build-time and run-time customizable
limits.
AUTHOR
Philip Hazel
Retired from University Computing Service
Cambridge, England.
REVISION
Last updated: 08 December 2021
Copyright (c) 1997-2021 University of Cambridge.
PCRE2 10.40 08 December 2021 PCRE2COMPAT(3)
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