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PCRECOMPAT(3)                                                    PCRECOMPAT(3)

       PCRE - Perl-compatible regular expressions


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

       1. PCRE has only a subset of Perl's UTF-8 and 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 sets its numerical variables from any such patterns that are matched before
       the assertion fails to match something (thereby succeeding), but only if the negative lookahead assertion  con-
       tains just one branch.

       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  pat-
       tern 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 some-
       what messy concept of surrogates."

       7. PCRE implements a simpler version of \X than Perl, which changed to make \X  match  what  Unicode  calls  an
       "extended  grapheme  cluster".  This  is more complicated than an extended Unicode sequence, which is what PCRE

       8. 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.

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

       10. 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.

       11. 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.

       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  PCRE
       it is set to "b".

       13. 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 captur-
       ing 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 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 whitespace between ( and ? but PCRE never does, even if
       the PCRE_EXTENDED option is set.

       15. 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  look-
       behind 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  posi-
       tion in the subject string.

       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 function (pcre_dfa_exec()) matches in a different way and is 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.


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


       Last updated: 14 November 2011
       Copyright (c) 1997-2011 University of Cambridge.