text.mod/regex.mod/pcre/doc/pcre2compat.3

.TH PCRE2COMPAT 3 "30 November 2023" "PCRE2 10.43"
.SH NAME
PCRE2 - Perl-compatible regular expressions (revised API)
.SH "DIFFERENCES BETWEEN PCRE2 AND PERL"
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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changing, the information may at times be out of date.
.P
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.
.P
2. PCRE2 has only a subset of Perl's Unicode support. Details of what it does
have are given in the
.\" HREF
\fBpcre2unicode\fP
.\"
page.
.P
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, \eb* , but these do not seem to have any use. PCRE2 does not allow
any kind of quantifier on non-lookaround assertions.
.P
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.
.P
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.
.P
6. The following Perl escape sequences are not supported: \eF, \el, \eL, \eu,
\eU, and \eN when followed by a character name. \eN on its own, matching a
non-newline character, and \eN{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, \eU and \eu are interpreted as ECMAScript
interprets them.
.P
7. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE2 is
built with Unicode support (the default). The properties that can be tested
with \ep and \eP 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
.\" HREF
\fBpcre2pattern\fP
.\"
documentation for details. The long synonyms for property names that Perl
supports (such as \ep{Letter}) are not supported by PCRE2, nor is it permitted
to prefix any of these properties with "Is".
.P
8. PCRE2 supports the \eQ...\eE 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 \eQ
and \eE which, its documentation says, "may lead to confusing results". PCRE2
treats a backslash between \eQ and \eE just like any other character. Note the
following examples:
.sp
    Pattern            PCRE2 matches     Perl matches
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.\" JOIN
    \eQabc$xyz\eE        abc$xyz           abc followed by the
                                           contents of $xyz
    \eQabc\e$xyz\eE       abc\e$xyz          abc\e$xyz
    \eQabc\eE\e$\eQxyz\eE   abc$xyz           abc$xyz
    \eQA\eB\eE            A\eB               A\eB
    \eQ\e\eE              \e                 \e\eE
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The \eQ...\eE sequence is recognized both inside and outside character classes
by both PCRE2 and Perl.
.P
9. Fairly obviously, PCRE2 does not support the (?{code}) and (??{code})
constructions. However, PCRE2 does have a "callout" feature, which allows an
external function to be called during pattern matching. See the
.\" HREF
\fBpcre2callout\fP
.\"
documentation for details.
.P
10. Subroutine calls (whether recursive or not) were treated as atomic groups
up to PCRE2 release 10.23, but from release 10.30 this changed, and
backtracking into subroutine calls is now supported, as in Perl.
.P
11. In PCRE2, if any of the backtracking control verbs are used in a group that
is called as a subroutine (whether or not recursively), their effect is
confined to that group; it does not extend to the surrounding pattern. This is
not always the case in Perl. In particular, if (*THEN) is present in a group
that is called as a subroutine, its action is limited to that group, even if
the group does not contain any | characters. Note that such groups are
processed as anchored at the point where they are tested.
.P
12. If a pattern contains more than one backtracking control verb, the first
one that is backtracked onto acts. For example, in the pattern
A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C
triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the
same as PCRE2, but there are cases where it differs.
.P
13. There are some differences that are concerned with the settings of captured
strings when part of a pattern is repeated. For example, matching "aba" against
the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE2 it is set to
"b".
.P
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.
.P
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.
.P
16. Perl, when in warning mode, gives warnings for character classes such as
[A-\ed] 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
17. In PCRE2, the upper/lower case character properties Lu and Ll are not
affected when case-independent matching is specified. For example, \ep{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), \ep{Lu} and \ep{Ll} match all
letters, regardless of case, when case independence is specified.
.P
18. From release 5.32.0, Perl locks out the use of \eK 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,
\eK is acted on when it occurs in positive assertions, but is ignored in
negative assertions.
.P
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:
.sp
(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.
.sp
(b) A backslash followed by a letter with no special meaning is faulted. (Perl
can be made to issue a warning.)
.sp
(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.
.sp
(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.
.sp
(e) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and PCRE2_NOTEMPTY_ATSTART
options have no Perl equivalents.
.sp
(f) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
by the PCRE2_BSR_ANYCRLF option.
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(g) The callout facility is PCRE2-specific. Perl supports codeblocks and
variable interpolation, but not general hooks on every match.
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(h) The partial matching facility is PCRE2-specific.
.sp
(i) The alternative matching function (\fBpcre2_dfa_match()\fP matches in a
different way and is not Perl-compatible.
.sp
(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.
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(k) PCRE2 supports non-atomic positive lookaround assertions. This is an
extension to the lookaround facilities. The default, Perl-compatible
lookarounds are atomic.
.sp
(l) There are three syntactical items in patterns that can refer to a capturing
group by number: back references such as \eg{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.
.P
20. Perl has different limits than PCRE2. See the
.\" HREF
\fBpcre2limit\fP
.\"
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.
.P
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.
.P
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)|\ez)/, which matches a sequence of any number of repeated
"abcd" substrings at the end of the subject.
.
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.SH AUTHOR
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Philip Hazel
Retired from University Computing Service
Cambridge, England.
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.SH REVISION
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Last updated: 30 November 2023
Copyright (c) 1997-2023 University of Cambridge.
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