PCRE2COMPAT(3)             Library Functions Manual             PCRE2COMPAT(3)

       PCRE2 - Perl-compatible regular expressions (revised API)


       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       continually changing, the information may at times be out of date.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

       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

       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

       (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

       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


       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


       Last updated: 08 December 2021
       Copyright (c) 1997-2021 University of Cambridge.

PCRE2 10.40                    08 December 2021                 PCRE2COMPAT(3)
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