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revision 93 by nigel, Sat Feb 24 21:41:42 2007 UTC revision 457 by ph10, Sat Oct 3 16:24:08 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 mainly with respect to  regular expressions. The differences described here are with respect to Perl
9  Perl 5.8, though PCRE version 7.0 contains some features that are expected to  5.10.
 be in the forthcoming Perl 5.10.  
10  .P  .P
11  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
12  it does have are given in the  it does have are given in the
# Line 46  encountered by PCRE, an error is generat Line 45  encountered by PCRE, an error is generat
45  built with Unicode character property support. The properties that can be  built with Unicode character property support. The properties that can be
46  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
47  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
48  and L&.  and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
49    Perl documentation says "Because Perl hides the need for the user to understand
50    the internal representation of Unicode characters, there is no need to
51    implement the somewhat messy concept of surrogates."
52  .P  .P
53  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
54  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 66  The \eQ...\eE sequence is recognized bot Line 68  The \eQ...\eE sequence is recognized bot
68  .P  .P
69  8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})  8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
70  constructions. However, there is support for recursive patterns. This is not  constructions. However, there is support for recursive patterns. This is not
71  available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"  available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
72  feature allows an external function to be called during pattern matching. See  feature allows an external function to be called during pattern matching. See
73  the  the
74  .\" HREF  .\" HREF
# Line 75  the Line 77  the
77  documentation for details.  documentation for details.
78  .P  .P
79  9. Subpatterns that are called recursively or as "subroutines" are always  9. Subpatterns that are called recursively or as "subroutines" are always
80  treated as atomic groups in PCRE. This is like Python, but unlike Perl.  treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
81    is a discussion of an example that explains this in more detail in the
82    .\" HTML <a href="pcrepattern.html#recursiondifference">
83    .\" </a>
84    section on recursion differences from Perl
85    .\"
86    in the
87    .\" HREF
88    \fBpcrepattern\fP
89    .\"
90    page.
91  .P  .P
92  10. There are some differences that are concerned with the settings of captured  10. There are some differences that are concerned with the settings of captured
93  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
94  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".
95  .P  .P
96  11. PCRE provides some extensions to the Perl regular expression facilities.  11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),
97  Perl 5.10 will include new features that are not in earlier versions, some of  (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an
98  which (such as named parentheses) have been in PCRE for some time. This list is  argument. PCRE does not support (*MARK).
99  with respect to Perl 5.10:  .P
100  .sp  12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
101  (a) Although lookbehind assertions must match fixed length strings, each  names is not as general as Perl's. This is a consequence of the fact the PCRE
102  alternative branch of a lookbehind assertion can match a different length of  works internally just with numbers, using an external table to translate
103  string. Perl requires them all to have the same length.  between numbers and names. The following are some specific differences:
104    .sp
105    (a) A pattern such as (?|(?<a>A)|(?<b)B), where the two capturing
106    parentheses have the same number but different names, is not supported, and
107    causes an error at compile time. If it were allowed, it would not be possible
108    to distinguish which parentheses matched, because both names map to capturing
109    subpattern number 1. To avoid this confusing situation, an error is given at
110    compile time.
111    .sp
112    (b) A condition test for a subpattern with a name that is duplicated gives
113    unpredictable results. For example, when the pattern
114    (?:(?<a>A)|(?<a>B))(?('a')...|...) is compiled (the PCRE_DUPNAMES option is
115    required), the condition test (?('a') is set to test whether subpattern 1 has
116    matched, ignoring subpattern 2, even though it has the same name.
117    .P
118    13. PCRE provides some extensions to the Perl regular expression facilities.
119    Perl 5.10 includes new features that are not in earlier versions of Perl, some
120    of which (such as named parentheses) have been in PCRE for some time. This list
121    is with respect to Perl 5.10:
122    .sp
123    (a) Although lookbehind assertions in PCRE must match fixed length strings,
124    each alternative branch of a lookbehind assertion can match a different length
125    of string. Perl requires them all to have the same length.
126  .sp  .sp
127  (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $  (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
128  meta-character matches only at the very end of the string.  meta-character matches only at the very end of the string.
129  .sp  .sp
130  (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
131  meaning is faulted. Otherwise, like Perl, the backslash is ignored. (Perl can  meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
132  be made to issue a warning.)  (Perl can be made to issue a warning.)
133  .sp  .sp
134  (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
135  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 104  question mark they are. Line 138  question mark they are.
138  (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
139  only at the first matching position in the subject string.  only at the first matching position in the subject string.
140  .sp  .sp
141  (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
142  options for \fBpcre_exec()\fP have no Perl equivalents.  PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
143    .sp
144    (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
145    by the PCRE_BSR_ANYCRLF option.
146  .sp  .sp
147  (g) The callout facility is PCRE-specific.  (h) The callout facility is PCRE-specific.
148  .sp  .sp
149  (h) The partial matching facility is PCRE-specific.  (i) The partial matching facility is PCRE-specific.
150  .sp  .sp
151  (i) Patterns compiled by PCRE can be saved and re-used at a later time, even on  (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
152  different hosts that have the other endianness.  different hosts that have the other endianness.
153  .sp  .sp
154  (j) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a  (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
155  different way and is not Perl-compatible.  different way and is not Perl-compatible.
156  .P  .sp
157  .in 0  (l) PCRE recognizes some special sequences such as (*CR) at the start of
158  Last updated: 28 November 2006  a pattern that set overall options that cannot be changed within the pattern.
159  .br  .
160  Copyright (c) 1997-2006 University of Cambridge.  .
161    .SH AUTHOR
162    .rs
163    .sp
164    .nf
165    Philip Hazel
166    University Computing Service
167    Cambridge CB2 3QH, England.
168    .fi
169    .
170    .
171    .SH REVISION
172    .rs
173    .sp
174    .nf
175    Last updated: 03 October 2009
176    Copyright (c) 1997-2009 University of Cambridge.
177    .fi
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