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

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Documentation and general text tidies in preparation for test release.
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 allows repeat quantifiers only on parenthesized assertions, but they do
33 not mean what you might think. For example, (?!a){3} does not assert that the
34 next three characters are not "a". It just asserts that the next character is
35 not "a" three times (in principle: PCRE optimizes this to run the assertion
36 just once). Perl allows repeat quantifiers on other assertions such as \b, but
37 these do not seem to have any use.
38 </P>
39 <P>
40 3. Capturing subpatterns that occur inside negative lookahead assertions are
41 counted, but their entries in the offsets vector are never set. Perl sets its
42 numerical variables from any such patterns that are matched before the
43 assertion fails to match something (thereby succeeding), but only if the
44 negative lookahead assertion contains just one branch.
45 </P>
46 <P>
47 4. Though binary zero characters are supported in the subject string, they are
48 not allowed in a pattern string because it is passed as a normal C string,
49 terminated by zero. The escape sequence \0 can be used in the pattern to
50 represent a binary zero.
51 </P>
52 <P>
53 5. The following Perl escape sequences are not supported: \l, \u, \L,
54 \U, and \N when followed by a character name or Unicode value. (\N on its
55 own, matching a non-newline character, is supported.) In fact these are
56 implemented by Perl's general string-handling and are not part of its pattern
57 matching engine. If any of these are encountered by PCRE, an error is
58 generated.
59 </P>
60 <P>
61 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
62 built with Unicode character property support. The properties that can be
63 tested with \p and \P are limited to the general category properties such as
64 Lu and Nd, script names such as Greek or Han, and the derived properties Any
65 and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
66 Perl documentation says "Because Perl hides the need for the user to understand
67 the internal representation of Unicode characters, there is no need to
68 implement the somewhat messy concept of surrogates."
69 </P>
70 <P>
71 7. PCRE implements a simpler version of \X than Perl, which changed to make
72 \X match what Unicode calls an "extended grapheme cluster". This is more
73 complicated than an extended Unicode sequence, which is what PCRE matches.
74 </P>
75 <P>
76 8. PCRE does support the \Q...\E escape for quoting substrings. Characters in
77 between are treated as literals. This is slightly different from Perl in that $
78 and @ are also handled as literals inside the quotes. In Perl, they cause
79 variable interpolation (but of course PCRE does not have variables). Note the
80 following examples:
81 <pre>
82 Pattern PCRE matches Perl matches
84 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
85 \Qabc\$xyz\E abc\$xyz abc\$xyz
86 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
87 </pre>
88 The \Q...\E sequence is recognized both inside and outside character classes.
89 </P>
90 <P>
91 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
92 constructions. However, there is support for recursive patterns. This is not
93 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
94 feature allows an external function to be called during pattern matching. See
95 the
96 <a href="pcrecallout.html"><b>pcrecallout</b></a>
97 documentation for details.
98 </P>
99 <P>
100 10. Subpatterns that are called recursively or as "subroutines" are always
101 treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
102 is a discussion of an example that explains this in more detail in the
103 <a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a>
104 in the
105 <a href="pcrepattern.html"><b>pcrepattern</b></a>
106 page.
107 </P>
108 <P>
109 11. There are some differences that are concerned with the settings of captured
110 strings when part of a pattern is repeated. For example, matching "aba" against
111 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
112 </P>
113 <P>
114 12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
115 names is not as general as Perl's. This is a consequence of the fact the PCRE
116 works internally just with numbers, using an external table to translate
117 between numbers and names. In particular, a pattern such as (?|(?&#60;a&#62;A)|(?&#60;b)B),
118 where the two capturing parentheses have the same number but different names,
119 is not supported, and causes an error at compile time. If it were allowed, it
120 would not be possible to distinguish which parentheses matched, because both
121 names map to capturing subpattern number 1. To avoid this confusing situation,
122 an error is given at compile time.
123 </P>
124 <P>
125 13. Perl recognizes comments in some places that PCRE does not, for example,
126 between the ( and ? at the start of a subpattern. If the /x modifier is set,
127 Perl allows whitespace between ( and ? but PCRE never does, even if the
128 PCRE_EXTENDED option is set.
129 </P>
130 <P>
131 14. PCRE provides some extensions to the Perl regular expression facilities.
132 Perl 5.10 includes new features that are not in earlier versions of Perl, some
133 of which (such as named parentheses) have been in PCRE for some time. This list
134 is with respect to Perl 5.10:
135 <br>
136 <br>
137 (a) Although lookbehind assertions in PCRE must match fixed length strings,
138 each alternative branch of a lookbehind assertion can match a different length
139 of string. Perl requires them all to have the same length.
140 <br>
141 <br>
142 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
143 meta-character matches only at the very end of the string.
144 <br>
145 <br>
146 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
147 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
148 (Perl can be made to issue a warning.)
149 <br>
150 <br>
151 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
152 inverted, that is, by default they are not greedy, but if followed by a
153 question mark they are.
154 <br>
155 <br>
156 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
157 only at the first matching position in the subject string.
158 <br>
159 <br>
161 PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents.
162 <br>
163 <br>
164 (g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
165 by the PCRE_BSR_ANYCRLF option.
166 <br>
167 <br>
168 (h) The callout facility is PCRE-specific.
169 <br>
170 <br>
171 (i) The partial matching facility is PCRE-specific.
172 <br>
173 <br>
174 (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
175 different hosts that have the other endianness.
176 <br>
177 <br>
178 (k) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
179 different way and is not Perl-compatible.
180 <br>
181 <br>
182 (l) PCRE recognizes some special sequences such as (*CR) at the start of
183 a pattern that set overall options that cannot be changed within the pattern.
184 </P>
185 <br><b>
187 </b><br>
188 <P>
189 Philip Hazel
190 <br>
191 University Computing Service
192 <br>
193 Cambridge CB2 3QH, England.
194 <br>
195 </P>
196 <br><b>
198 </b><br>
199 <P>
200 Last updated: 24 July 2011
201 <br>
202 Copyright &copy; 1997-2011 University of Cambridge.
203 <br>
204 <p>
205 Return to the <a href="index.html">PCRE index page</a>.
206 </p>


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