This document describes the differences in the ways that PCRE2 and Perl
       handle regular expressions. The differences  described  here  are  with
       respect  to Perl versions 5.26, but as both Perl and PCRE2 are continu-
       ally changing, the information may sometimes be out of date.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}), but these do not seem to have any use.

       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).

       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&.  PCRE2 does support the Cs (surrogate) property,
       which  Perl  does  not; the Perl documentation says "Because Perl hides
       the need for the user to understand the internal representation of Uni-
       code  characters, there is no need to implement the somewhat messy con-
       cept of surrogates."

       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

       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. 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  lim-
       ited  to  that  group, even if the group does not contain any | charac-
       ters. 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.  Most  backtracking  verbs in assertions have their normal actions.
       They are not confined to the assertion.

       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
       unset, 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

       (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 that  set  overall  options  that  cannot  be
       changed within the pattern.

       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
       University Computing Service
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