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revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 450 by ph10, Wed Sep 16 10:56:40 2009 UTC
# Line 5  PCRE - Perl-compatible regular expressio Line 5  PCRE - Perl-compatible regular expressio
5  .rs  .rs
6  .sp  .sp
7  This document describes the differences in the ways that PCRE and Perl handle  This document describes the differences in the ways that PCRE and Perl handle
8  regular expressions. The differences described here are with respect to Perl  regular expressions. The differences described here are mainly with respect to
9  5.8.  Perl 5.8, though PCRE versions 7.0 and later contain some features that are
10    in Perl 5.10.
11  .P  .P
12  1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what  1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
13  it does have are given in the  it does have are given in the
# Line 45  encountered by PCRE, an error is generat Line 46  encountered by PCRE, an error is generat
46  built with Unicode character property support. The properties that can be  built with Unicode character property support. The properties that can be
47  tested with \ep and \eP are limited to the general category properties such as  tested with \ep and \eP are limited to the general category properties such as
48  Lu and Nd, script names such as Greek or Han, and the derived properties Any  Lu and Nd, script names such as Greek or Han, and the derived properties Any
49  and L&.  and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
50    Perl documentation says "Because Perl hides the need for the user to understand
51    the internal representation of Unicode characters, there is no need to
52    implement the somewhat messy concept of surrogates."
53  .P  .P
54  7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in  7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
55  between are treated as literals. This is slightly different from Perl in that $  between are treated as literals. This is slightly different from Perl in that $
# Line 63  following examples: Line 67  following examples:
67  .sp  .sp
68  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
69  .P  .P
70  8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})  8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
71  constructions. However, there is support for recursive patterns using the  constructions. However, there is support for recursive patterns. This is not
72  non-Perl items (?R), (?number), and (?P>name). Also, the PCRE "callout" feature  available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"
73  allows an external function to be called during pattern matching. See the  feature allows an external function to be called during pattern matching. See
74    the
75  .\" HREF  .\" HREF
76  \fBpcrecallout\fP  \fBpcrecallout\fP
77  .\"  .\"
78  documentation for details.  documentation for details.
79  .P  .P
80  9. There are some differences that are concerned with the settings of captured  9. Subpatterns that are called recursively or as "subroutines" are always
81    treated as atomic groups in PCRE. This is like Python, but unlike Perl.
82    .P
83    10. There are some differences that are concerned with the settings of captured
84  strings when part of a pattern is repeated. For example, matching "aba" against  strings when part of a pattern is repeated. For example, matching "aba" against
85  the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".  the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
86  .P  .P
87  10. PCRE provides some extensions to the Perl regular expression facilities:  11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),
88    (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an
89    argument. PCRE does not support (*MARK).
90    .P
91    12. PCRE provides some extensions to the Perl regular expression facilities.
92    Perl 5.10 will include new features that are not in earlier versions, some of
93    which (such as named parentheses) have been in PCRE for some time. This list is
94    with respect to Perl 5.10:
95  .sp  .sp
96  (a) Although lookbehind assertions must match fixed length strings, each  (a) Although lookbehind assertions must match fixed length strings, each
97  alternative branch of a lookbehind assertion can match a different length of  alternative branch of a lookbehind assertion can match a different length of
# Line 86  string. Perl requires them all to have t Line 101  string. Perl requires them all to have t
101  meta-character matches only at the very end of the string.  meta-character matches only at the very end of the string.
102  .sp  .sp
103  (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special  (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
104  meaning is faulted. Otherwise, like Perl, the backslash is ignored. (Perl can  meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
105  be made to issue a warning.)  (Perl can be made to issue a warning.)
106  .sp  .sp
107  (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is  (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
108  inverted, that is, by default they are not greedy, but if followed by a  inverted, that is, by default they are not greedy, but if followed by a
# Line 96  question mark they are. Line 111  question mark they are.
111  (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried  (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
112  only at the first matching position in the subject string.  only at the first matching position in the subject string.
113  .sp  .sp
114  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
115  options for \fBpcre_exec()\fP have no Perl equivalents.  PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
116  .sp  .sp
117  (g) The (?R), (?number), and (?P>name) constructs allows for recursive pattern  (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
118  matching (Perl can do this using the (?p{code}) construct, which PCRE cannot  by the PCRE_BSR_ANYCRLF option.
 support.)  
119  .sp  .sp
120  (h) PCRE supports named capturing substrings, using the Python syntax.  (h) The callout facility is PCRE-specific.
121  .sp  .sp
122  (i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java  (i) The partial matching facility is PCRE-specific.
 package.  
123  .sp  .sp
124  (j) The (R) condition, for testing recursion, is a PCRE extension.  (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
125    different hosts that have the other endianness.
126  .sp  .sp
127  (k) The callout facility is PCRE-specific.  (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
128    different way and is not Perl-compatible.
129  .sp  .sp
130  (l) The partial matching facility is PCRE-specific.  (l) PCRE recognizes some special sequences such as (*CR) at the start of
131    a pattern that set overall options that cannot be changed within the pattern.
132    .
133    .
134    .SH AUTHOR
135    .rs
136  .sp  .sp
137  (m) Patterns compiled by PCRE can be saved and re-used at a later time, even on  .nf
138  different hosts that have the other endianness.  Philip Hazel
139    University Computing Service
140    Cambridge CB2 3QH, England.
141    .fi
142    .
143    .
144    .SH REVISION
145    .rs
146  .sp  .sp
147  (n) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a  .nf
148  different way and is not Perl-compatible.  Last updated: 16 September 2009
149  .P  Copyright (c) 1997-2009 University of Cambridge.
150  .in 0  .fi
 Last updated: 06 June 2006  
 .br  
 Copyright (c) 1997-2006 University of Cambridge.  
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