2014-09-29 18:45:37 +02:00
|
|
|
<html>
|
|
|
|
|
<head>
|
|
|
|
|
<title>pcre2compat specification</title>
|
|
|
|
|
</head>
|
|
|
|
|
<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
|
|
|
|
|
<h1>pcre2compat man page</h1>
|
|
|
|
|
<p>
|
|
|
|
|
Return to the <a href="index.html">PCRE2 index page</a>.
|
|
|
|
|
</p>
|
|
|
|
|
<p>
|
|
|
|
|
This page is part of the PCRE2 HTML documentation. It was generated
|
|
|
|
|
automatically from the original man page. If there is any nonsense in it,
|
|
|
|
|
please consult the man page, in case the conversion went wrong.
|
|
|
|
|
<br>
|
|
|
|
|
<br><b>
|
|
|
|
|
DIFFERENCES BETWEEN PCRE2 AND PERL
|
|
|
|
|
</b><br>
|
|
|
|
|
<P>
|
2020-10-07 18:27:20 +02:00
|
|
|
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 5.32.0, but as both Perl and PCRE2 are continually changing, the
|
|
|
|
|
information may at times be out of date.
|
2014-09-29 18:45:37 +02:00
|
|
|
</P>
|
|
|
|
|
<P>
|
|
|
|
|
1. PCRE2 has only a subset of Perl's Unicode support. Details of what it does
|
|
|
|
|
have are given in the
|
|
|
|
|
<a href="pcre2unicode.html"><b>pcre2unicode</b></a>
|
|
|
|
|
page.
|
|
|
|
|
</P>
|
|
|
|
|
<P>
|
2017-03-31 18:49:33 +02:00
|
|
|
2. 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
|
2018-06-17 16:13:28 +02:00
|
|
|
character is not "a" three times (in principle; PCRE2 optimizes this to run the
|
2017-03-31 18:49:33 +02:00
|
|
|
assertion just once). Perl allows some repeat quantifiers on other assertions,
|
2020-10-07 18:27:20 +02:00
|
|
|
for example, \b* (but not \b{3}, though oddly it does allow ^{3}), but these
|
2020-11-06 18:27:35 +01:00
|
|
|
do not seem to have any use. PCRE2 does not allow any kind of quantifier on
|
2020-10-07 18:27:20 +02:00
|
|
|
non-lookaround assertions.
|
2014-09-29 18:45:37 +02:00
|
|
|
</P>
|
|
|
|
|
<P>
|
2019-02-06 19:11:36 +01:00
|
|
|
3. Capture groups that occur inside negative lookaround assertions are counted,
|
|
|
|
|
but their entries in the offsets vector are set only when a negative assertion
|
2020-11-06 18:27:35 +01:00
|
|
|
is a condition that has a matching branch (that is, the condition is false).
|
2020-10-07 18:27:20 +02:00
|
|
|
Perl may set such capture groups in other circumstances.
|
2014-09-29 18:45:37 +02:00
|
|
|
</P>
|
|
|
|
|
<P>
|
2018-08-13 13:57:09 +02:00
|
|
|
4. The following Perl escape sequences are not supported: \F, \l, \L, \u,
|
2018-07-28 18:23:24 +02:00
|
|
|
\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
|
2014-09-29 18:45:37 +02:00
|
|
|
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
|
2019-02-12 18:50:19 +01:00
|
|
|
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.
|
2014-09-29 18:45:37 +02:00
|
|
|
</P>
|
|
|
|
|
<P>
|
|
|
|
|
5. The Perl escape sequences \p, \P, and \X are supported only if PCRE2 is
|
2017-03-31 18:49:33 +02:00
|
|
|
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, and the derived properties Any and L&.
|
2020-10-07 18:27:20 +02:00
|
|
|
Both PCRE2 and Perl support the Cs (surrogate) property, but in PCRE2 its use
|
|
|
|
|
is limited. See the
|
|
|
|
|
<a href="pcre2pattern.html"><b>pcre2pattern</b></a>
|
|
|
|
|
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".
|
2014-09-29 18:45:37 +02:00
|
|
|
</P>
|
|
|
|
|
<P>
|
2018-07-28 18:23:24 +02:00
|
|
|
6. 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 (but of course 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:
|
2014-09-29 18:45:37 +02:00
|
|
|
<pre>
|
2018-07-28 18:23:24 +02:00
|
|
|
Pattern PCRE2 matches Perl matches
|
2014-09-29 18:45:37 +02:00
|
|
|
|
|
|
|
|
\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
|
2018-07-28 18:23:24 +02:00
|
|
|
\QA\B\E A\B A\B
|
|
|
|
|
\Q\\E \ \\E
|
2014-09-29 18:45:37 +02:00
|
|
|
</pre>
|
2020-11-06 18:27:35 +01:00
|
|
|
The \Q...\E sequence is recognized both inside and outside character classes
|
2020-10-07 18:27:20 +02:00
|
|
|
by both PCRE2 and Perl.
|
2014-09-29 18:45:37 +02:00
|
|
|
</P>
|
|
|
|
|
<P>
|
|
|
|
|
7. Fairly obviously, PCRE2 does not support the (?{code}) and (??{code})
|
2018-06-17 16:13:28 +02:00
|
|
|
constructions. However, PCRE2 does have a "callout" feature, which allows an
|
|
|
|
|
external function to be called during pattern matching. See the
|
2014-09-29 18:45:37 +02:00
|
|
|
<a href="pcre2callout.html"><b>pcre2callout</b></a>
|
|
|
|
|
documentation for details.
|
|
|
|
|
</P>
|
|
|
|
|
<P>
|
2017-03-31 18:49:33 +02:00
|
|
|
8. 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
|
2017-07-19 18:04:15 +02:00
|
|
|
into subroutine calls is now supported, as in Perl.
|
2014-09-29 18:45:37 +02:00
|
|
|
</P>
|
|
|
|
|
<P>
|
2020-10-07 18:27:20 +02:00
|
|
|
9. 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.
|
2014-09-29 18:45:37 +02:00
|
|
|
</P>
|
|
|
|
|
<P>
|
|
|
|
|
10. 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
|
2016-11-22 16:37:02 +01:00
|
|
|
same as PCRE2, but there are cases where it differs.
|
2014-09-29 18:45:37 +02:00
|
|
|
</P>
|
|
|
|
|
<P>
|
2020-10-07 18:27:20 +02:00
|
|
|
11. There are some differences that are concerned with the settings of captured
|
2014-09-29 18:45:37 +02:00
|
|
|
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".
|
|
|
|
|
</P>
|
|
|
|
|
<P>
|
2020-10-07 18:27:20 +02:00
|
|
|
12. PCRE2's handling of duplicate capture group numbers and names is not as
|
2019-02-06 19:11:36 +01:00
|
|
|
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
|
2020-10-07 18:27:20 +02:00
|
|
|
names. In particular, a pattern such as (?|(?<a>A)|(?<b>B)), where the two
|
2019-02-06 19:11:36 +01:00
|
|
|
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.
|
2014-09-29 18:45:37 +02:00
|
|
|
</P>
|
|
|
|
|
<P>
|
2020-10-07 18:27:20 +02:00
|
|
|
13. Perl used to recognize comments in some places that PCRE2 does not, for
|
2019-02-06 19:11:36 +01:00
|
|
|
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.
|
2014-09-29 18:45:37 +02:00
|
|
|
</P>
|
|
|
|
|
<P>
|
2020-10-07 18:27:20 +02:00
|
|
|
14. Perl, when in warning mode, gives warnings for character classes such as
|
2014-09-29 18:45:37 +02:00
|
|
|
[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.
|
|
|
|
|
</P>
|
|
|
|
|
<P>
|
2020-10-07 18:27:20 +02:00
|
|
|
15. In PCRE2, the upper/lower case character properties Lu and Ll are not
|
2014-09-29 18:45:37 +02:00
|
|
|
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;
|
2020-10-07 18:27:20 +02:00
|
|
|
in the release at the time of writing (5.32), \p{Lu} and \p{Ll} match all
|
2014-09-29 18:45:37 +02:00
|
|
|
letters, regardless of case, when case independence is specified.
|
|
|
|
|
</P>
|
|
|
|
|
<P>
|
2020-11-06 18:27:35 +01:00
|
|
|
16. From release 5.32.0, Perl locks out the use of \K in lookaround
|
2021-08-30 17:59:34 +02:00
|
|
|
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.
|
2020-10-07 18:27:20 +02:00
|
|
|
</P>
|
|
|
|
|
<P>
|
2014-09-29 18:45:37 +02:00
|
|
|
17. PCRE2 provides some extensions to the Perl regular expression facilities.
|
2020-10-07 18:27:20 +02:00
|
|
|
Perl 5.10 included new features that were not in earlier versions of Perl, some
|
2017-03-31 18:49:33 +02:00
|
|
|
of which (such as named parentheses) were in PCRE2 for some time before. This
|
2020-10-07 18:27:20 +02:00
|
|
|
list is with respect to Perl 5.32:
|
2014-09-29 18:45:37 +02:00
|
|
|
<br>
|
|
|
|
|
<br>
|
|
|
|
|
(a) Although lookbehind assertions in PCRE2 must match fixed length strings,
|
2020-10-07 18:27:20 +02:00
|
|
|
each alternative toplevel branch of a lookbehind assertion can match a
|
|
|
|
|
different length of string. Perl requires them all to have the same length.
|
2014-09-29 18:45:37 +02:00
|
|
|
<br>
|
|
|
|
|
<br>
|
2018-06-17 16:13:28 +02:00
|
|
|
(b) From PCRE2 10.23, backreferences to groups of fixed length are supported
|
2016-11-22 16:37:02 +01:00
|
|
|
in lookbehinds, provided that there is no possibility of referencing a
|
|
|
|
|
non-unique number or name. Perl does not support backreferences in lookbehinds.
|
|
|
|
|
<br>
|
|
|
|
|
<br>
|
|
|
|
|
(c) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the $
|
2014-09-29 18:45:37 +02:00
|
|
|
meta-character matches only at the very end of the string.
|
|
|
|
|
<br>
|
|
|
|
|
<br>
|
2016-11-22 16:37:02 +01:00
|
|
|
(d) A backslash followed by a letter with no special meaning is faulted. (Perl
|
2014-09-29 18:45:37 +02:00
|
|
|
can be made to issue a warning.)
|
|
|
|
|
<br>
|
|
|
|
|
<br>
|
2016-11-22 16:37:02 +01:00
|
|
|
(e) If PCRE2_UNGREEDY is set, the greediness of the repetition quantifiers is
|
2014-09-29 18:45:37 +02:00
|
|
|
inverted, that is, by default they are not greedy, but if followed by a
|
|
|
|
|
question mark they are.
|
|
|
|
|
<br>
|
|
|
|
|
<br>
|
2016-11-22 16:37:02 +01:00
|
|
|
(f) PCRE2_ANCHORED can be used at matching time to force a pattern to be tried
|
2014-09-29 18:45:37 +02:00
|
|
|
only at the first matching position in the subject string.
|
|
|
|
|
<br>
|
|
|
|
|
<br>
|
2017-04-17 17:39:09 +02:00
|
|
|
(g) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and PCRE2_NOTEMPTY_ATSTART
|
|
|
|
|
options have no Perl equivalents.
|
2014-09-29 18:45:37 +02:00
|
|
|
<br>
|
|
|
|
|
<br>
|
2016-11-22 16:37:02 +01:00
|
|
|
(h) The \R escape sequence can be restricted to match only CR, LF, or CRLF
|
2014-09-29 18:45:37 +02:00
|
|
|
by the PCRE2_BSR_ANYCRLF option.
|
|
|
|
|
<br>
|
|
|
|
|
<br>
|
2017-04-17 17:39:09 +02:00
|
|
|
(i) The callout facility is PCRE2-specific. Perl supports codeblocks and
|
|
|
|
|
variable interpolation, but not general hooks on every match.
|
2014-09-29 18:45:37 +02:00
|
|
|
<br>
|
|
|
|
|
<br>
|
2016-11-22 16:37:02 +01:00
|
|
|
(j) The partial matching facility is PCRE2-specific.
|
2014-09-29 18:45:37 +02:00
|
|
|
<br>
|
|
|
|
|
<br>
|
2016-11-22 16:37:02 +01:00
|
|
|
(k) The alternative matching function (<b>pcre2_dfa_match()</b> matches in a
|
2014-09-29 18:45:37 +02:00
|
|
|
different way and is not Perl-compatible.
|
|
|
|
|
<br>
|
|
|
|
|
<br>
|
2017-04-17 17:39:09 +02:00
|
|
|
(l) PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT) at
|
2020-10-07 18:27:20 +02:00
|
|
|
the start of a pattern. These set overall options that cannot be changed within
|
2017-04-17 17:39:09 +02:00
|
|
|
the pattern.
|
2019-07-13 13:12:03 +02:00
|
|
|
<br>
|
|
|
|
|
<br>
|
2019-10-17 18:39:38 +02:00
|
|
|
(m) PCRE2 supports non-atomic positive lookaround assertions. This is an
|
2019-07-13 13:12:03 +02:00
|
|
|
extension to the lookaround facilities. The default, Perl-compatible
|
|
|
|
|
lookarounds are atomic.
|
2017-04-18 14:32:52 +02:00
|
|
|
</P>
|
|
|
|
|
<P>
|
|
|
|
|
18. The Perl /a modifier restricts /d numbers to pure ascii, and the /aa
|
|
|
|
|
modifier restricts /i case-insensitive matching to pure ascii, ignoring Unicode
|
|
|
|
|
rules. This separation cannot be represented with PCRE2_UCP.
|
|
|
|
|
</P>
|
|
|
|
|
<P>
|
2017-04-17 17:39:09 +02:00
|
|
|
19. Perl has different limits than PCRE2. See the
|
|
|
|
|
<a href="pcre2limit.html"><b>pcre2limit</b></a>
|
|
|
|
|
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.
|
2014-09-29 18:45:37 +02:00
|
|
|
</P>
|
|
|
|
|
<br><b>
|
|
|
|
|
AUTHOR
|
|
|
|
|
</b><br>
|
|
|
|
|
<P>
|
|
|
|
|
Philip Hazel
|
|
|
|
|
<br>
|
2021-08-30 17:59:34 +02:00
|
|
|
Retired from University Computing Service
|
2014-09-29 18:45:37 +02:00
|
|
|
<br>
|
2014-11-21 17:45:06 +01:00
|
|
|
Cambridge, England.
|
2014-09-29 18:45:37 +02:00
|
|
|
<br>
|
|
|
|
|
</P>
|
|
|
|
|
<br><b>
|
|
|
|
|
REVISION
|
|
|
|
|
</b><br>
|
|
|
|
|
<P>
|
2021-08-30 17:59:34 +02:00
|
|
|
Last updated: 30 August 2021
|
2014-09-29 18:45:37 +02:00
|
|
|
<br>
|
2021-08-30 17:59:34 +02:00
|
|
|
Copyright © 1997-2021 University of Cambridge.
|
2014-09-29 18:45:37 +02:00
|
|
|
<br>
|
|
|
|
|
<p>
|
|
|
|
|
Return to the <a href="index.html">PCRE2 index page</a>.
|
|
|
|
|
</p>
|
