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 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 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_COM-
       PAT  option  is set, \U and \u are interpreted as JavaScript interprets

       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&. 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 representa-
       tion  of Unicode characters, there is no need to implement the somewhat
       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 matches.

       8. PCRE does support the \Q...\E escape for quoting substrings. Charac-
       The  \Q...\E  sequence  is recognized both inside and outside character

       9. 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 pat-
       tern matching. See the pcrecallout documentation for details.

       10. Subpatterns that are called as subroutines (whether or  not  recur-
       sively)  are  always  treated  as  atomic  groups in PCRE. This is like
       Python, but unlike Perl.  Captured values that are set outside  a  sub-
       routine  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  any of the backtracking control verbs are used in an assertion
       or 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. There is one exception to this: the name
       from a *(MARK), (*PRUNE), or (*THEN) that is encountered in a  success-
       ful  positive  assertion  is passed back when a match succeeds (compare
       capturing parentheses in assertions). Note that  such  subpatterns  are
       processed as anchored at the point where they are tested.

       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  sub-
       pattern 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 ta-
       ble  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 cap-
       turing 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 white space between ( and ? but PCRE never
       does, even if the PCRE_EXTENDED option is set.

       15. PCRE provides some extensions to the Perl regular expression facil-
       ities.   Perl  5.10  includes new features that are not in earlier ver-
       sions 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:

       (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.

       and  PCRE_NO_AUTO_CAPTURE  options for pcre_exec() have no Perl equiva-

       (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()   and
       pcre16_dfa_exec()) match in a different way and are  not  Perl-compati-

       (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


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


       Last updated: 01 June 2012
       Copyright (c) 1997-2012 University of Cambridge.

PCRE 8.30                       08 January 2012                  PCRECOMPAT(3)
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