NAME

       PCRE - Perl-compatible regular expressions

DIFFERENCES BETWEEN PCRE AND PERL

       This  document  describes  the differences in the ways that PCRE and Perl handle regular expressions. The
       differences described here are with respect to Perl versions 5.10 and above.

       1. PCRE has only a subset of Perl's Unicode support. Details of what  it  does  have  are  given  in  the
       pcreunicode page.

       2.  PCRE  allows repeat quantifiers only 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: PCRE optimizes this to run the assertion
       just once). Perl allows repeat quantifiers on other assertions such as \b, but these do not seem to  have
       any use.

       3.  Capturing  subpatterns that occur inside negative lookahead assertions are counted, but their entries
       in the offsets vector are never set. Perl sometimes (but not always) sets its  numerical  variables  from
       inside negative assertions.

       4.  Though  binary zero characters are supported in the subject string, they are not allowed in a pattern
       string because it is passed as a normal C string, terminated by zero. The escape sequence \0 can be  used
       in the pattern to represent a binary zero.

       5.  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  PCRE,  an  error  is  generated  by  default.  However,  if  the
       PCRE_JAVASCRIPT_COMPAT option is set, \U and \u are interpreted as JavaScript interprets them.

       6.  The  Perl  escape sequences \p, \P, and \X are supported only if PCRE is built with Unicode character
       property support. 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&.
       PCRE 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."

       7. PCRE 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 PCRE does not have variables). Note the
       following examples:

           Pattern            PCRE 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.

       8.  Fairly obviously, PCRE does not support the (?{code}) and (??{code}) constructions. However, there is
       support for recursive patterns. This is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE
       "callout"  feature  allows an external function to be called during pattern matching. See the pcrecallout
       documentation for details.

       9. Subpatterns that are called as subroutines (whether or not recursively) are always treated  as  atomic
       groups  in PCRE. This is like Python, but unlike Perl.  Captured values that are set outside a subroutine
       call can be reference from inside in PCRE, but not in Perl. There is a  discussion  that  explains  these
       differences in more detail in the section on recursion differences from Perl in the pcrepattern page.

       10.  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.

       11. 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 PCRE, but there
       are examples where it differs.

       12.  Most  backtracking  verbs  in  assertions  have  their  normal actions. They are not confined to the
       assertion.

       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 PCRE it is set to "b".

       14. PCRE'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 PCRE 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.

       15. Perl recognizes comments in some places that PCRE does not, for example, between the ( and ?  at  the
       start of a subpattern. If the /x modifier is set, Perl allows white space between ( and ? (though current
       Perls warn that this is deprecated) but PCRE never does, even if the PCRE_EXTENDED option is set.

       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. PCRE has no warning features, so it gives an error in these cases
       because they are almost certainly user mistakes.

       17. In PCRE, 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 (5.16),  \p{Lu}  and  \p{Ll}  match  all  letters,
       regardless of case, when case independence is specified.

       18.  PCRE  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) have been in
       PCRE for some time. This list is with respect to Perl 5.10:

       (a)  Although lookbehind assertions in PCRE 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) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta-character matches only at the
       very end of the string.

       (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special meaning is faulted. Otherwise,
       like Perl, the backslash is quietly ignored.  (Perl can be made to issue a warning.)

       (d)  If  PCRE_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.

       (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried only at the first  matching
       position in the subject string.

       (f)  The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and PCRE_NO_AUTO_CAPTURE options
       for pcre_exec() have no Perl equivalents.

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

       (h) The callout facility is PCRE-specific.

       (i) The partial matching facility is PCRE-specific.

       (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on different hosts that have
       the other endianness. However, this does  not  apply  to  optimized  data  created  by  the  just-in-time
       compiler.

       (k)  The alternative matching functions (pcre_dfa_exec(), pcre16_dfa_exec() and pcre32_dfa_exec(),) match
       in a different way and are not Perl-compatible.

       (l) PCRE recognizes some special sequences such as (*CR) at the start  of  a  pattern  that  set  overall
       options that cannot be changed within the pattern.

AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

       Last updated: 10 November 2013
       Copyright (c) 1997-2013 University of Cambridge.