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 5.32.0, but as both Perl and PCRE2 are
       continually changing, the information may at times be out of date.

       1. PCRE2 has only a subset of Perl's Unicode support. Details  of  what
       it does have are given in the pcre2unicode page.

       2.  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
       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  not
       \b{3},  though oddly it does allow ^{3}), but these do not seem to have
       any use. PCRE2 does not allow any kind of quantifier on  non-lookaround

       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 is a condition that has a matching branch (that is,
       the condition is false).  Perl may set such  capture  groups  in  other

       4.  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.

       5. 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, and  the  derived
       properties  Any and L&.  Both PCRE2 and Perl support the Cs (surrogate)
       property, but in PCRE2 its use is limited. See the  pcre2pattern  docu-
       mentation  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".

       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 documen-
       tation  says, "may lead to confusing results". PCRE2 treats a backslash
       between \Q and \E just like any other  character.  Note  the  following

           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.

       7.  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.

       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 into subroutine calls is now supported, as in Perl.

       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 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.

       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 same as PCRE2, but there are cases where it differs.

       11. 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".

       12.  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.

       13. 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.

       14.  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.

       15. 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 (5.32), \p{Lu} and
       \p{Ll} match all letters, regardless of case, when case independence is

       16.  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.

       17.  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 5.32:

       (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 requires them all to have  the
       same 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.

       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

       19. 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: 30 August 2021
       Copyright (c) 1997-2021 University of Cambridge.

PCRE2 10.38                     30 August 2021                  PCRE2COMPAT(3)
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