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revision 75 by nigel, Sat Feb 24 21:40:37 2007 UTC revision 457 by ph10, Sat Oct 3 16:24:08 2009 UTC
# Line 1  Line 1 
1  .TH PCRE 3  .TH PCRECOMPAT 3
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH "DIFFERENCES BETWEEN PCRE AND PERL"  .SH "DIFFERENCES BETWEEN PCRE AND PERL"
# Line 6  PCRE - Perl-compatible regular expressio Line 6  PCRE - Perl-compatible regular expressio
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 with respect to Perl
9  5.8.  5.10.
10  .P  .P
11  1. PCRE does not have full UTF-8 support. Details of what it does have are  1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
12  given in the  it does have are given in the
13  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
14  .\" </a>  .\" </a>
15  section on UTF-8 support  section on UTF-8 support
# Line 44  encountered by PCRE, an error is generat Line 44  encountered by PCRE, an error is generat
44  6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is  6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
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.  Lu and Nd, script names such as Greek or Han, and the derived properties Any
48    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 62  following examples: Line 66  following examples:
66  .sp  .sp
67  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
68  .P  .P
69  8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})  8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
70  constructions. However, there is support for recursive patterns using the  constructions. However, there is support for recursive patterns. This is not
71  non-Perl items (?R), (?number), and (?P>name). Also, the PCRE "callout" feature  available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
72  allows an external function to be called during pattern matching. See the  feature allows an external function to be called during pattern matching. See
73    the
74  .\" HREF  .\" HREF
75  \fBpcrecallout\fP  \fBpcrecallout\fP
76  .\"  .\"
77  documentation for details.  documentation for details.
78  .P  .P
79  9. There are some differences that are concerned with the settings of captured  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. 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
92    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  10. PCRE provides some extensions to the Perl regular expression facilities:  11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),
97  .sp  (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an
98  (a) Although lookbehind assertions must match fixed length strings, each  argument. PCRE does not support (*MARK).
99  alternative branch of a lookbehind assertion can match a different length of  .P
100  string. Perl requires them all to have the same length.  12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
101    names is not as general as Perl's. This is a consequence of the fact the PCRE
102    works internally just with numbers, using an external table to translate
103    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.  meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
132    (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 94  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  .sp
144  (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
145  matching (Perl can do this using the (?p{code}) construct, which PCRE cannot  by the PCRE_BSR_ANYCRLF option.
 support.)  
146  .sp  .sp
147  (h) PCRE supports named capturing substrings, using the Python syntax.  (h) The callout facility is PCRE-specific.
148  .sp  .sp
149  (i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java  (i) The partial matching facility is PCRE-specific.
 package.  
150  .sp  .sp
151  (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
152    different hosts that have the other endianness.
153  .sp  .sp
154  (k) The callout facility is PCRE-specific.  (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
155    different way and is not Perl-compatible.
156  .sp  .sp
157  (l) The partial matching facility is PCRE-specific.  (l) PCRE recognizes some special sequences such as (*CR) at the start of
158    a pattern that set overall options that cannot be changed within the pattern.
159    .
160    .
161    .SH AUTHOR
162    .rs
163  .sp  .sp
164  (m) Patterns compiled by PCRE can be saved and re-used at a later time, even on  .nf
165  different hosts that have the other endianness.  Philip Hazel
166  .P  University Computing Service
167  .in 0  Cambridge CB2 3QH, England.
168  Last updated: 09 September 2004  .fi
169  .br  .
170  Copyright (c) 1997-2004 University of Cambridge.  .
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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