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NAME | DIFFERENCES BETWEEN PCRE2 AND PERL | AUTHOR | REVISION | COLOPHON |
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PCRE2COMPAT(3) Library Functions Manual PCRE2COMPAT(3)
PCRE2 - Perl-compatible regular expressions (revised API)
This document describes some of the known differences in the ways
that PCRE2 and Perl handle regular expressions. The differences
described here are with respect to Perl version 5.38.0, 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at times be out of date.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 sequence. 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
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; 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. If a braced quantifier such as {1,2} appears where there is
nothing to repeat (for example, at the start of a branch), PCRE2
raises an error whereas Perl treats the quantifier characters as
literal.
5. 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.
6. 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, matching 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.
7. 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, the derived properties Any
and LC (synonym L&), script names such as Greek or Han,
Bidi_Class, Bidi_Control, and a few binary properties. 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{Letter}) are not supported by PCRE2, nor is it permitted to
prefix any of these properties with "Is".
8. 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$xyz
\QA\B\E A\B A\B
\Q\\E \ \\E
The \Q...\E sequence is recognized both inside and outside
character classes by both PCRE2 and Perl.
9. Fairly obviously, PCRE2 does not support the (?{code}) and
(??{code}) constructions. However, PCRE2 does have a "callout"
feature, which allows an external function to be called during
pattern matching. See the pcre2callout documentation for details.
10. Subroutine calls (whether recursive or not) were treated as
atomic groups up to PCRE2 release 10.23, but from release 10.30
this changed, and backtracking into subroutine calls is now
supported, as in Perl.
11. In PCRE2, if any of the backtracking control verbs are used
in a group that is called as a subroutine (whether or not
recursively), their effect is confined to that group; it does not
extend to the surrounding pattern. This is not always the case in
Perl. In particular, if (*THEN) is present in a group that is
called as a subroutine, its action is limited to that group, even
if the group does not contain any | characters. Note that such
groups are processed as anchored at the point where they are
tested.
12. If a pattern contains more than one backtracking control
verb, the first one that is backtracked onto acts. For example,
in the pattern A(*COMMIT)B(*PRUNE)C a failure in B triggers
(*COMMIT), but a failure in C triggers (*PRUNE). Perl's behaviour
is more complex; in many cases it is the same as PCRE2, but there
are cases where it differs.
13. There are some differences that are concerned with the
settings of captured strings when part of a pattern is repeated.
For example, matching "aba" against the pattern /^(a(b)?)+$/ in
Perl leaves $2 unset, but in PCRE2 it is set to "b".
14. 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 translate 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.
15. 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.
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. PCRE2 has no warning features, so it gives
an error in these cases because they are almost certainly user
mistakes.
17. 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 (5.38), \p{Lu} and \p{Ll} match all letters, regardless
of case, when case independence is specified.
18. From release 5.32.0, Perl locks out the use of \K in
lookaround assertions. 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.
19. PCRE2 provides some extensions to the Perl regular expression
facilities. 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 5.38:
(a) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not
set, the $ meta-character matches only at the very end of the
string.
(b) A backslash followed by a letter with no special meaning is
faulted. (Perl can be made to issue a warning.)
(c) If PCRE2_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.
(d) 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.
(e) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and
PCRE2_NOTEMPTY_ATSTART options have no Perl equivalents.
(f) The \R escape sequence can be restricted to match only CR,
LF, or CRLF by the PCRE2_BSR_ANYCRLF option.
(g) The callout facility is PCRE2-specific. Perl supports
codeblocks and variable interpolation, but not general hooks on
every match.
(h) The partial matching facility is PCRE2-specific.
(i) The alternative matching function (pcre2_dfa_match() matches
in a different way and is not Perl-compatible.
(j) 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.
(k) PCRE2 supports non-atomic positive lookaround assertions.
This is an extension to the lookaround facilities. The default,
Perl-compatible lookarounds are atomic.
(l) There are three syntactical items in patterns that can refer
to a capturing group by number: back references such as \g{2},
subroutine calls such as (?3), and condition references such as
(?(4)...). PCRE2 supports relative group numbers such as +2 and
-4 in all three cases. Perl supports both plus and minus for
subroutine calls, but only minus for back references, and no
relative numbering at all for conditions.
20. Perl has different limits than PCRE2. See the pcre2limit
documentation for details. Perl went with 5.10 from recursion to
iteration keeping 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.
21. Unlike Perl, PCRE2 doesn't have character set modifiers and
specially no way to set characters by context just like Perl's
"/d". A regular expression using PCRE2_UTF and PCRE2_UCP will use
similar rules to Perl's "/u"; something closer to "/a" could be
selected by adding other PCRE2_EXTRA_ASCII* options on top.
22. Some recursive patterns that Perl diagnoses as infinite
recursions can be handled by PCRE2, either by the interpreter or
the JIT. An example is /(?:|(?0)abcd)(?(R)|\z)/, which matches a
sequence of any number of repeated "abcd" substrings at the end
of the subject.
Philip Hazel
Retired from University Computing Service
Cambridge, England.
Last updated: 30 November 2023
Copyright (c) 1997-2023 University of Cambridge.
This page is part of the PCRE (Perl Compatible Regular
Expressions) project. Information about the project can be found
at ⟨http://www.pcre.org/⟩. If you have a bug report for this
manual page, see
⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩. This page was
obtained from the tarball fetched from
⟨https://github.com/PhilipHazel/pcre2.git⟩ on 2024-06-14. If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to [email protected]
PCRE2 10.43 30 November 2023 PCRE2COMPAT(3)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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