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1 <html>
2 <head>
3 <title>pcrecompat specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcrecompat man page</h1>
7 <p>
8 Return to the <a href="index.html">PCRE index page</a>.
9 </p>
10 <p>
11 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 <br>
15 <br><b>
16 DIFFERENCES BETWEEN PCRE AND PERL
17 </b><br>
18 <P>
19 This document describes the differences in the ways that PCRE and Perl handle
20 regular expressions. The differences described here are mainly with respect to
21 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 </P>
24 <P>
25 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 <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 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 3. Capturing subpatterns that occur inside negative lookahead assertions are
40 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 4. Though binary zero characters are supported in the subject string, they are
47 not allowed in a pattern string because it is passed as a normal C string,
48 terminated by zero. The escape sequence \0 can be used in the pattern to
49 represent a binary zero.
50 </P>
51 <P>
52 5. The following Perl escape sequences are not supported: \l, \u, \L,
53 \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 </P>
57 <P>
58 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 Lu and Nd, script names such as Greek or Han, and the derived properties Any
62 and L&.
63 </P>
64 <P>
65 7. PCRE does support the \Q...\E escape for quoting substrings. Characters in
66 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
73 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
74 \Qabc\$xyz\E abc\$xyz abc\$xyz
75 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
76 </pre>
77 The \Q...\E sequence is recognized both inside and outside character classes.
78 </P>
79 <P>
80 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
81 constructions. However, there is support for recursive patterns. This is not
82 available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"
83 feature allows an external function to be called during pattern matching. See
84 the
85 <a href="pcrecallout.html"><b>pcrecallout</b></a>
86 documentation for details.
87 </P>
88 <P>
89 9. Subpatterns that are called recursively or as "subroutines" are always
90 treated as atomic groups in PCRE. This is like Python, but unlike Perl.
91 </P>
92 <P>
93 10. There are some differences that are concerned with the settings of captured
94 strings when part of a pattern is repeated. For example, matching "aba" against
95 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
96 </P>
97 <P>
98 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 <br>
103 <br>
104 (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 <br>
108 <br>
109 (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 <br>
112 <br>
113 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
114 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
115 (Perl can be made to issue a warning.)
116 <br>
117 <br>
118 (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 <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 (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 <br>
130 <br>
131 (g) The callout facility is PCRE-specific.
132 <br>
133 <br>
134 (h) The partial matching facility is PCRE-specific.
135 <br>
136 <br>
137 (i) Patterns compiled by PCRE can be saved and re-used at a later time, even on
138 different hosts that have the other endianness.
139 <br>
140 <br>
141 (j) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
142 different way and is not Perl-compatible.
143 </P>
144 <br><b>
145 AUTHOR
146 </b><br>
147 <P>
148 Philip Hazel
149 <br>
150 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 Last updated: 13 June 2007
160 <br>
161 Copyright &copy; 1997-2007 University of Cambridge.
162 <br>
163 <p>
164 Return to the <a href="index.html">PCRE index page</a>.
165 </p>

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