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       continually changing, the information may sometimes be out of date.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. Capturing subpatterns 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: \l, \u, \L, \U,  and  \N  when  followed  by  a
       character name or Unicode value. (\N on its own, matching a non-newline character, is supported.) In fact
       these 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 the
       PCRE2_ALT_BSUX option 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  does  support  the \Q...\E escape for quoting substrings. Characters in between are treated as
       literals. 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).  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

       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, there is
       support  PCRE2's  "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 subpattern that is called as a subroutine
       (whether or not recursively), their effect is confined to that subpattern; 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  subpatterns  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 subpattern numbers and duplicate subpattern 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  capturing parentheses 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  parentheses
       matched,  because  both names map to capturing subpattern 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 subpattern. 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, back references 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.

       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: 18 April 2017
       Copyright (c) 1997-2017 University of Cambridge.