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       both  Perl  and  PCRE2  are  continually changing, the information may sometimes be out of
       date.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.