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           This  document describes the differences in the ways that PCRE and Perl
           handle regular expressions. The differences described here  are  mainly
           with  respect  to  Perl 5.8, though PCRE versions 7.0 and later contain
           some features that are expected to be in the forthcoming Perl 5.10.
           1. PCRE has only a subset of Perl's UTF-8 and Unicode support.  Details
           of  what  it does have are given in the section on UTF-8 support in the
           main pcre page.
           2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
           permits  them,  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.
           3.  Capturing  subpatterns  that occur inside negative lookahead asser-
           tions 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 succeed-
           ing),  but  only  if the negative lookahead assertion contains just one
           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 nor-
           mal 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. In fact these are implemented by Perl's general string-han-
           dling  and are not part of its pattern matching engine. If any of these
           are encountered by PCRE, an error is generated.
           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  prop-
           erties  such  as  Lu and Nd, script names such as Greek or Han, and the
           derived properties Any and L&.
           7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
           ters  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$xyz
           captured strings when part of  a  pattern  is  repeated.  For  example,
           matching  "aba"  against  the  pattern  /^(a(b)?)+$/  in Perl leaves $2
           unset, but in PCRE it is set to "b".
           11.  PCRE  does  support  Perl  5.10's  backtracking  verbs  (*ACCEPT),
           (*FAIL),  (*F),  (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in
           the forms without an  argument.  PCRE  does  not  support  (*MARK).  If
           (*ACCEPT)  is within capturing parentheses, PCRE does not set that cap-
           ture group; this is different to Perl.
           12. PCRE provides some extensions to the Perl regular expression facil-
           ities.   Perl  5.10  will  include new features that are not in earlier
           versions, 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  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 spe-
           cial 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  quanti-
           fiers is inverted, that is, by default they are not greedy, but if fol-
           lowed 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.
           TURE 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.
           (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

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