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1 nigel 63 <html>
2     <head>
3     <title>pcrecompat specification</title>
4     </head>
5     <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 nigel 75 <h1>pcrecompat man page</h1>
7     <p>
8     Return to the <a href="index.html">PCRE index page</a>.
9     </p>
10 ph10 111 <p>
11 nigel 75 This page is part of the PCRE HTML documentation. It was generated automatically
12     from the original man page. If there is any nonsense in it, please consult the
13     man page, in case the conversion went wrong.
14 ph10 111 <br>
15 nigel 75 <br><b>
16     DIFFERENCES BETWEEN PCRE AND PERL
17     </b><br>
18 nigel 63 <P>
19     This document describes the differences in the ways that PCRE and Perl handle
20 nigel 93 regular expressions. The differences described here are mainly with respect to
21 ph10 182 Perl 5.8, though PCRE versions 7.0 and later contain some features that are
22     expected to be in the forthcoming Perl 5.10.
23 nigel 63 </P>
24     <P>
25 nigel 87 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
26     it does have are given in the
27 nigel 73 <a href="pcre.html#utf8support">section on UTF-8 support</a>
28     in the main
29     <a href="pcre.html"><b>pcre</b></a>
30     page.
31     </P>
32     <P>
33     2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
34 nigel 63 them, but they do not mean what you might think. For example, (?!a){3} does
35     not assert that the next three characters are not "a". It just asserts that the
36     next character is not "a" three times.
37     </P>
38     <P>
39 nigel 73 3. Capturing subpatterns that occur inside negative lookahead assertions are
40 nigel 63 counted, but their entries in the offsets vector are never set. Perl sets its
41     numerical variables from any such patterns that are matched before the
42     assertion fails to match something (thereby succeeding), but only if the
43     negative lookahead assertion contains just one branch.
44     </P>
45     <P>
46 nigel 73 4. Though binary zero characters are supported in the subject string, they are
47 nigel 63 not allowed in a pattern string because it is passed as a normal C string,
48 nigel 75 terminated by zero. The escape sequence \0 can be used in the pattern to
49 nigel 63 represent a binary zero.
50     </P>
51     <P>
52 nigel 73 5. The following Perl escape sequences are not supported: \l, \u, \L,
53 nigel 75 \U, and \N. In fact these are implemented by Perl's general string-handling
54     and are not part of its pattern matching engine. If any of these are
55     encountered by PCRE, an error is generated.
56 nigel 63 </P>
57     <P>
58 nigel 75 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
59     built with Unicode character property support. The properties that can be
60     tested with \p and \P are limited to the general category properties such as
61 nigel 87 Lu and Nd, script names such as Greek or Han, and the derived properties Any
62     and L&.
63 nigel 75 </P>
64     <P>
65     7. PCRE does support the \Q...\E escape for quoting substrings. Characters in
66 nigel 63 between are treated as literals. This is slightly different from Perl in that $
67     and @ are also handled as literals inside the quotes. In Perl, they cause
68     variable interpolation (but of course PCRE does not have variables). Note the
69     following examples:
70     <pre>
71     Pattern PCRE matches Perl matches
72 nigel 75
73     \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
74 nigel 63 \Qabc\$xyz\E abc\$xyz abc\$xyz
75     \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
76 nigel 75 </pre>
77 nigel 73 The \Q...\E sequence is recognized both inside and outside character classes.
78 nigel 63 </P>
79     <P>
80 nigel 93 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
81     constructions. However, there is support for recursive patterns. This is not
82     available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"
83     feature allows an external function to be called during pattern matching. See
84     the
85 nigel 75 <a href="pcrecallout.html"><b>pcrecallout</b></a>
86     documentation for details.
87 nigel 63 </P>
88     <P>
89 nigel 93 9. Subpatterns that are called recursively or as "subroutines" are always
90     treated as atomic groups in PCRE. This is like Python, but unlike Perl.
91     </P>
92     <P>
93     10. There are some differences that are concerned with the settings of captured
94 nigel 63 strings when part of a pattern is repeated. For example, matching "aba" against
95     the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
96     </P>
97     <P>
98 nigel 93 11. PCRE provides some extensions to the Perl regular expression facilities.
99     Perl 5.10 will include new features that are not in earlier versions, some of
100     which (such as named parentheses) have been in PCRE for some time. This list is
101     with respect to Perl 5.10:
102 nigel 75 <br>
103     <br>
104 nigel 63 (a) Although lookbehind assertions must match fixed length strings, each
105     alternative branch of a lookbehind assertion can match a different length of
106     string. Perl requires them all to have the same length.
107 nigel 75 <br>
108     <br>
109 nigel 63 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
110     meta-character matches only at the very end of the string.
111 nigel 75 <br>
112     <br>
113     (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
114 ph10 182 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
115     (Perl can be made to issue a warning.)
116 nigel 75 <br>
117     <br>
118 nigel 63 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
119     inverted, that is, by default they are not greedy, but if followed by a
120     question mark they are.
121 nigel 75 <br>
122     <br>
123     (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
124     only at the first matching position in the subject string.
125     <br>
126     <br>
127 nigel 63 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
128     options for <b>pcre_exec()</b> have no Perl equivalents.
129 nigel 75 <br>
130     <br>
131 nigel 93 (g) The callout facility is PCRE-specific.
132 nigel 75 <br>
133     <br>
134 nigel 93 (h) The partial matching facility is PCRE-specific.
135 nigel 75 <br>
136     <br>
137 nigel 93 (i) Patterns compiled by PCRE can be saved and re-used at a later time, even on
138 nigel 75 different hosts that have the other endianness.
139 nigel 77 <br>
140     <br>
141 nigel 93 (j) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
142 nigel 77 different way and is not Perl-compatible.
143 nigel 63 </P>
144 ph10 99 <br><b>
145     AUTHOR
146     </b><br>
147 nigel 63 <P>
148 ph10 99 Philip Hazel
149 nigel 63 <br>
150 ph10 99 University Computing Service
151     <br>
152     Cambridge CB2 3QH, England.
153     <br>
154     </P>
155     <br><b>
156     REVISION
157     </b><br>
158     <P>
159 ph10 182 Last updated: 13 June 2007
160 ph10 99 <br>
161     Copyright &copy; 1997-2007 University of Cambridge.
162     <br>
163 nigel 75 <p>
164     Return to the <a href="index.html">PCRE index page</a>.
165     </p>

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