PCRECOMPAT(3)						     Library Functions Manual						     PCRECOMPAT(3)

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 , 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  suc-
       ceeding), but only if the negative lookahead assertion contains 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  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  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 matches.

       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$xyzE	      abc$xyz		abc followed by the
						  contents of $xyz
	   Qabc$xyzE       abc$xyz		abc$xyz
	   QabcE$QxyzE   abc$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 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.

       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 con-
       tain 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 pat-
       tern 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.

       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  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:

       (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  qui-
       etly 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 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.

       (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() and pcre16_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: 08 Januray 2012
       Copyright (c) 1997-2012 University of Cambridge.

																     PCRECOMPAT(3)