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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>
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 with respect to Perl
21 5.8.
22 </P>
23 <P>
24 1. PCRE does not have full UTF-8 support. Details of what it does have are
25 given in the
26 <a href="pcre.html#utf8support">section on UTF-8 support</a>
27 in the main
28 <a href="pcre.html"><b>pcre</b></a>
29 page.
30 </P>
31 <P>
32 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
33 them, but they do not mean what you might think. For example, (?!a){3} does
34 not assert that the next three characters are not "a". It just asserts that the
35 next character is not "a" three times.
36 </P>
37 <P>
38 3. Capturing subpatterns that occur inside negative lookahead assertions are
39 counted, but their entries in the offsets vector are never set. Perl sets its
40 numerical variables from any such patterns that are matched before the
41 assertion fails to match something (thereby succeeding), but only if the
42 negative lookahead assertion contains just one branch.
43 </P>
44 <P>
45 4. Though binary zero characters are supported in the subject string, they are
46 not allowed in a pattern string because it is passed as a normal C string,
47 terminated by zero. The escape sequence \0 can be used in the pattern to
48 represent a binary zero.
49 </P>
50 <P>
51 5. The following Perl escape sequences are not supported: \l, \u, \L,
52 \U, and \N. In fact these are implemented by Perl's general string-handling
53 and are not part of its pattern matching engine. If any of these are
54 encountered by PCRE, an error is generated.
55 </P>
56 <P>
57 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
58 built with Unicode character property support. The properties that can be
59 tested with \p and \P are limited to the general category properties such as
60 Lu and Nd.
61 </P>
62 <P>
63 7. PCRE does support the \Q...\E escape for quoting substrings. Characters in
64 between are treated as literals. This is slightly different from Perl in that $
65 and @ are also handled as literals inside the quotes. In Perl, they cause
66 variable interpolation (but of course PCRE does not have variables). Note the
67 following examples:
68 <pre>
69 Pattern PCRE matches Perl matches
71 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
72 \Qabc\$xyz\E abc\$xyz abc\$xyz
73 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
74 </pre>
75 The \Q...\E sequence is recognized both inside and outside character classes.
76 </P>
77 <P>
78 8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
79 constructions. However, there is support for recursive patterns using the
80 non-Perl items (?R), (?number), and (?P&#62;name). Also, the PCRE "callout" feature
81 allows an external function to be called during pattern matching. See the
82 <a href="pcrecallout.html"><b>pcrecallout</b></a>
83 documentation for details.
84 </P>
85 <P>
86 9. There are some differences that are concerned with the settings of captured
87 strings when part of a pattern is repeated. For example, matching "aba" against
88 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
89 </P>
90 <P>
91 10. PCRE provides some extensions to the Perl regular expression facilities:
92 <br>
93 <br>
94 (a) Although lookbehind assertions must match fixed length strings, each
95 alternative branch of a lookbehind assertion can match a different length of
96 string. Perl requires them all to have the same length.
97 <br>
98 <br>
99 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
100 meta-character matches only at the very end of the string.
101 <br>
102 <br>
103 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
104 meaning is faulted.
105 <br>
106 <br>
107 (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
109 question mark they are.
110 <br>
111 <br>
112 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
113 only at the first matching position in the subject string.
114 <br>
115 <br>
117 options for <b>pcre_exec()</b> have no Perl equivalents.
118 <br>
119 <br>
120 (g) The (?R), (?number), and (?P&#62;name) constructs allows for recursive pattern
121 matching (Perl can do this using the (?p{code}) construct, which PCRE cannot
122 support.)
123 <br>
124 <br>
125 (h) PCRE supports named capturing substrings, using the Python syntax.
126 <br>
127 <br>
128 (i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java
129 package.
130 <br>
131 <br>
132 (j) The (R) condition, for testing recursion, is a PCRE extension.
133 <br>
134 <br>
135 (k) The callout facility is PCRE-specific.
136 <br>
137 <br>
138 (l) The partial matching facility is PCRE-specific.
139 <br>
140 <br>
141 (m) Patterns compiled by PCRE can be saved and re-used at a later time, even on
142 different hosts that have the other endianness.
143 <br>
144 <br>
145 (n) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
146 different way and is not Perl-compatible.
147 </P>
148 <P>
149 Last updated: 28 February 2005
150 <br>
151 Copyright &copy; 1997-2005 University of Cambridge.
152 <p>
153 Return to the <a href="index.html">PCRE index page</a>.
154 </p>

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