NAME

       PCRE2 - Perl-compatible regular expressions (revised API)

DIFFERENCES BETWEEN PCRE2 AND PERL

       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       be out of date.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 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}, 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 assertions.

       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 circumstances.

       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&.  Both PCRE2 and Perl support the Cs
       (surrogate) property, but in PCRE2 its use is limited. See the pcre2pattern  documentation
       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 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 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 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. 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 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.

       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 literals. 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
       specified.

       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  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. 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 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
       Retired from University Computing Service
       Cambridge, England.

REVISION

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