ViewVC logotype

Contents of /code/trunk/doc/html/pcrecompat.html

Parent Directory Parent Directory | Revision Log Revision Log

Revision 567 - (show annotations)
Sat Nov 6 17:10:00 2010 UTC (8 years, 9 months ago) by ph10
File MIME type: text/html
File size: 7499 byte(s)
Test for ridiculous values of starting offsets; tidy UTF-8 code.
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 versions 5.10 and above.
22 </P>
23 <P>
24 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
25 it does have are 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, script names such as Greek or Han, and the derived properties Any
61 and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
62 Perl documentation says "Because Perl hides the need for the user to understand
63 the internal representation of Unicode characters, there is no need to
64 implement the somewhat messy concept of surrogates."
65 </P>
66 <P>
67 7. PCRE does support the \Q...\E escape for quoting substrings. Characters in
68 between are treated as literals. This is slightly different from Perl in that $
69 and @ are also handled as literals inside the quotes. In Perl, they cause
70 variable interpolation (but of course PCRE does not have variables). Note the
71 following examples:
72 <pre>
73 Pattern PCRE matches Perl matches
75 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
76 \Qabc\$xyz\E abc\$xyz abc\$xyz
77 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
78 </pre>
79 The \Q...\E sequence is recognized both inside and outside character classes.
80 </P>
81 <P>
82 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
83 constructions. However, there is support for recursive patterns. This is not
84 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
85 feature allows an external function to be called during pattern matching. See
86 the
87 <a href="pcrecallout.html"><b>pcrecallout</b></a>
88 documentation for details.
89 </P>
90 <P>
91 9. Subpatterns that are called recursively or as "subroutines" are always
92 treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
93 is a discussion of an example that explains this in more detail in the
94 <a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a>
95 in the
96 <a href="pcrepattern.html"><b>pcrepattern</b></a>
97 page.
98 </P>
99 <P>
100 10. There are some differences that are concerned with the settings of captured
101 strings when part of a pattern is repeated. For example, matching "aba" against
102 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
103 </P>
104 <P>
105 11. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
106 names is not as general as Perl's. This is a consequence of the fact the PCRE
107 works internally just with numbers, using an external table to translate
108 between numbers and names. In particular, a pattern such as (?|(?&#60;a&#62;A)|(?&#60;b)B),
109 where the two capturing parentheses have the same number but different names,
110 is not supported, and causes an error at compile time. If it were allowed, it
111 would not be possible to distinguish which parentheses matched, because both
112 names map to capturing subpattern number 1. To avoid this confusing situation,
113 an error is given at compile time.
114 </P>
115 <P>
116 12. Perl recognizes comments in some places that PCRE doesn't, for example,
117 between the ( and ? at the start of a subpattern.
118 </P>
119 <P>
120 13. PCRE provides some extensions to the Perl regular expression facilities.
121 Perl 5.10 includes new features that are not in earlier versions of Perl, some
122 of which (such as named parentheses) have been in PCRE for some time. This list
123 is with respect to Perl 5.10:
124 <br>
125 <br>
126 (a) Although lookbehind assertions in PCRE must match fixed length strings,
127 each alternative branch of a lookbehind assertion can match a different length
128 of string. Perl requires them all to have the same length.
129 <br>
130 <br>
131 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
132 meta-character matches only at the very end of the string.
133 <br>
134 <br>
135 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
136 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
137 (Perl can be made to issue a warning.)
138 <br>
139 <br>
140 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
141 inverted, that is, by default they are not greedy, but if followed by a
142 question mark they are.
143 <br>
144 <br>
145 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
146 only at the first matching position in the subject string.
147 <br>
148 <br>
150 PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents.
151 <br>
152 <br>
153 (g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
154 by the PCRE_BSR_ANYCRLF option.
155 <br>
156 <br>
157 (h) The callout facility is PCRE-specific.
158 <br>
159 <br>
160 (i) The partial matching facility is PCRE-specific.
161 <br>
162 <br>
163 (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
164 different hosts that have the other endianness.
165 <br>
166 <br>
167 (k) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
168 different way and is not Perl-compatible.
169 <br>
170 <br>
171 (l) PCRE recognizes some special sequences such as (*CR) at the start of
172 a pattern that set overall options that cannot be changed within the pattern.
173 </P>
174 <br><b>
176 </b><br>
177 <P>
178 Philip Hazel
179 <br>
180 University Computing Service
181 <br>
182 Cambridge CB2 3QH, England.
183 <br>
184 </P>
185 <br><b>
187 </b><br>
188 <P>
189 Last updated: 31 October 2010
190 <br>
191 Copyright &copy; 1997-2010 University of Cambridge.
192 <br>
193 <p>
194 Return to the <a href="index.html">PCRE index page</a>.
195 </p>


Name Value
svn:eol-style native
svn:keywords "Author Date Id Revision Url"

  ViewVC Help
Powered by ViewVC 1.1.5