pcre2compat(3) — Linux manual page


PCRE2COMPAT(3)          Library Functions Manual          PCRE2COMPAT(3)

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       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       at times be out of date.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

       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

       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

       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

       (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.

       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.

AUTHOR         top

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.

REVISION         top

       Last updated: 30 November 2023
       Copyright (c) 1997-2023 University of Cambridge.

COLOPHON         top

       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 2023-12-22.  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 man-pages@man7.org

PCRE2 10.43                 30 November 2023              PCRE2COMPAT(3)