NAME

       PCRE2 - Perl-compatible regular expressions (revised API)

DIFFERENCES BETWEEN PCRE2 AND PERL

       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
       continually changing, the information may sometimes be out of date.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 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 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, 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.

       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 Unicode characters, there
       is no need to implement the somewhat messy concept 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  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.

       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.  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.

       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
       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.

       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 literals. 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 (5.24), \p{Lu} and \p{Ll} match all letters,
       regardless of case, when case independence is specified.

       17. PCRE2 provides some extensions to the Perl regular expression facilities.   Perl  5.10  includes  new
       features  that  are  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 5.26:

       (a) Although lookbehind assertions in PCRE2 must match fixed length strings, each alternative 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 supported in lookbehinds, provided
       that there is no possibility  of  referencing  a  non-unique  number  or  name.  Perl  does  not  support
       backreferences 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 quantifiers is inverted, that is, by
       default they are not greedy, but if followed 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.

       (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
       PCRE2_UCP.

       19.  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.

AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.

REVISION

       Last updated: 13 July 2019
       Copyright (c) 1997-2019 University of Cambridge.

PCRE2 10.34                                       13 July 2019                                    PCRE2COMPAT(3)