NAME

       PCRE2 - Perl-compatible regular expressions (revised API)

DIFFERENCES BETWEEN PCRE2 AND PERL

       This  document  describes  some  of  the known differences in the ways that PCRE2 and Perl handle regular
       expressions. The differences described here are with respect to Perl version 5.38.0, but as both Perl and
       PCRE2 are continually changing, the information may at times be out of date.

       1. When PCRE2_DOTALL (equivalent  to  Perl's  /s  qualifier)  is  not  set,  the  behaviour  of  the  '.'
       metacharacter  differs  from  Perl.  In PCRE2, '.' matches the next character unless it is the start of a
       newline sequence. This means that, if the newline setting is CR, CRLF, or NUL, '.' will  match  the  code
       point  LF  (0x0A) in ASCII/Unicode environments, and NL (either 0x15 or 0x25) when using EBCDIC. In Perl,
       '.' appears never to match LF, even when 0x0A is not a newline indicator.

       2. PCRE2 has only a subset of Perl's Unicode support. Details of what it  does  have  are  given  in  the
       pcre2unicode page.

       3.  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
       these  do  not  seem  to  have  any  use.  PCRE2  does not allow any kind of quantifier on non-lookaround
       assertions.

       4. If a braced quantifier such as {1,2} appears where there is nothing to repeat  (for  example,  at  the
       start of a branch), PCRE2 raises an error whereas Perl treats the quantifier characters as literal.

       5.  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.

       6. 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.

       7.  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, the derived properties Any and Lc (synonym L&), script names such as Greek
       or Han, Bidi_Class, Bidi_Control, and a few binary  properties.  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".

       8.  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 (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.
       Another  difference  from  Perl  is  that  any  appearance  of  \Q or \E inside what might otherwise be a
       quantifier causes PCRE2 not to recognize the sequence as a quantifier. Perl recognizes  a  quantifier  if
       (redundantly)  either  of  the  numbers  is  inside \Q...\E, but not if the separating comma is. When not
       recognized as a quantifier a sequence such as {\Q1\E,2} is treated as the literal string "{1,2}".

       9. 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.

       10. 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.

       11. 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.
       PCRE2 also confines all control verbs within atomic assertions, again  including  (*THEN)  in  assertions
       with only one branch.

       12. 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.

       13.  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".

       14.  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.

       15. 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.

       16. 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.

       17. In PCRE2, until release 10.45, the upper/lower case character properties Lu and Ll were not  affected
       when case-independent matching was specified. Perl has changed in this respect, and PCRE2 has now changed
       to  match.  When  caseless matching is in force, Lu, Ll, and Lt (title case) are all treated as Lc (cased
       letter).

       18. 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.

       19.  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 5.38:

       (a)  If  PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the $ meta-character matches only at
       the very end of the string.

       (b) A backslash followed by a letter with no special meaning is faulted. (Perl can be  made  to  issue  a
       warning.)

       (c)  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.

       (d) 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.

       (e) The PCRE2_NOTBOL, PCRE2_NOTEOL,  PCRE2_NOTEMPTY  and  PCRE2_NOTEMPTY_ATSTART  options  have  no  Perl
       equivalents.

       (f)  The  \R  escape  sequence  can  be restricted to match only CR, LF, or CRLF by the PCRE2_BSR_ANYCRLF
       option.

       (g) The callout facility is PCRE2-specific. Perl supports codeblocks and variable interpolation, but  not
       general hooks on every match.

       (h) The partial matching facility is PCRE2-specific.

       (i)  The  alternative  matching  function  (pcre2_dfa_match() matches in a different way and is not Perl-
       compatible.

       (j) 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.

       (k)  PCRE2  supports  non-atomic  positive  lookaround assertions. This is an extension to the lookaround
       facilities. The default, Perl-compatible lookarounds are atomic.

       (l) There are three syntactical items in patterns that can refer to a capturing  group  by  number:  back
       references  such  as  \g{2},  subroutine  calls such as (?3), and condition references such as (?(4)...).
       PCRE2 supports relative group numbers such as +2 and -4 in all three cases. Perl supports both  plus  and
       minus  for  subroutine  calls,  but  only minus for back references, and no relative numbering at all for
       conditions.

       (m) The scan substring assertion (syntax (*scs:(n)...)) is a PCRE2 extension that  is  not  available  in
       Perl.

       20.  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.

       21. Unlike Perl, PCRE2 doesn't have character set modifiers and specially no way  to  set  characters  by
       context  just like Perl's "/d". A regular expression using PCRE2_UTF and PCRE2_UCP will use similar rules
       to Perl's "/u"; something closer to "/a" could be selected by adding other PCRE2_EXTRA_ASCII* options  on
       top.

       22. Some recursive patterns that Perl diagnoses as infinite recursions can be handled by PCRE2, either by
       the  interpreter  or  the  JIT.  An  example is /(?:|(?0)abcd)(?(R)|\z)/, which matches a sequence of any
       number of repeated "abcd" substrings at the end of the subject.

       23. Both PCRE2 and Perl error when \x{ escapes are invalid, but  Perl  tries  to  recover  and  prints  a
       warning if the problem was that an invalid hexadecimal digit was found, since PCRE2 doesn't have warnings
       it returns an error instead.  Additionally, Perl accepts \x{} and generates NUL unlike PCRE2.

       24.  From  release  10.45,  PCRE2  gives an error if \x is not followed by a hexadecimal digit or a curly
       bracket. It used to interpret this as the NUL character. Perl still generates  NUL,  but  warns  when  in
       warning mode in most cases.

AUTHOR

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.

REVISION

       Last updated: 02 October 2024
       Copyright (c) 1997-2024 University of Cambridge.

PCRE2 10.45                                      02 October 2024                                  PCRE2COMPAT(3)