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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 versions 5.10 and above.
22 </P>
23 <P>
24 1. PCRE has only a subset of Perl's Unicode support. Details of what it does
25 have are given in the
26 <a href="pcreunicode.html"><b>pcreunicode</b></a>
27 page.
28 </P>
29 <P>
30 2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do
31 not mean what you might think. For example, (?!a){3} does not assert that the
32 next three characters are not "a". It just asserts that the next character is
33 not "a" three times (in principle: PCRE optimizes this to run the assertion
34 just once). Perl allows repeat quantifiers on other assertions such as \b, but
35 these do not seem to have any use.
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 when followed by a character name or Unicode value. (\N on its
53 own, matching a non-newline character, is supported.) In fact these are
54 implemented by Perl's general string-handling and are not part of its pattern
55 matching engine. If any of these are encountered by PCRE, an error is
56 generated by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
57 \U and \u are interpreted as JavaScript interprets them.
58 </P>
59 <P>
60 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
61 built with Unicode character property support. The properties that can be
62 tested with \p and \P are limited to the general category properties such as
63 Lu and Nd, script names such as Greek or Han, and the derived properties Any
64 and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
65 Perl documentation says "Because Perl hides the need for the user to understand
66 the internal representation of Unicode characters, there is no need to
67 implement the somewhat messy concept of surrogates."
68 </P>
69 <P>
70 7. PCRE does support the \Q...\E escape for quoting substrings. Characters in
71 between are treated as literals. This is slightly different from Perl in that $
72 and @ are also handled as literals inside the quotes. In Perl, they cause
73 variable interpolation (but of course PCRE does not have variables). Note the
74 following examples:
75 <pre>
76 Pattern PCRE matches Perl matches
78 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
79 \Qabc\$xyz\E abc\$xyz abc\$xyz
80 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
81 </pre>
82 The \Q...\E sequence is recognized both inside and outside character classes.
83 </P>
84 <P>
85 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
86 constructions. However, there is support for recursive patterns. This is not
87 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
88 feature allows an external function to be called during pattern matching. See
89 the
90 <a href="pcrecallout.html"><b>pcrecallout</b></a>
91 documentation for details.
92 </P>
93 <P>
94 9. Subpatterns that are called as subroutines (whether or not recursively) are
95 always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
96 Captured values that are set outside a subroutine call can be reference from
97 inside in PCRE, but not in Perl. There is a discussion that explains these
98 differences in more detail in the
99 <a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a>
100 in the
101 <a href="pcrepattern.html"><b>pcrepattern</b></a>
102 page.
103 </P>
104 <P>
105 10. If any of the backtracking control verbs are used in an assertion or in a
106 subpattern that is called as a subroutine (whether or not recursively), their
107 effect is confined to that subpattern; it does not extend to the surrounding
108 pattern. This is not always the case in Perl. In particular, if (*THEN) is
109 present in a group that is called as a subroutine, its action is limited to
110 that group, even if the group does not contain any | characters. There is one
111 exception to this: the name from a *(MARK), (*PRUNE), or (*THEN) that is
112 encountered in a successful positive assertion <i>is</i> passed back when a
113 match succeeds (compare capturing parentheses in assertions). Note that such
114 subpatterns are processed as anchored at the point where they are tested.
115 </P>
116 <P>
117 11. There are some differences that are concerned with the settings of captured
118 strings when part of a pattern is repeated. For example, matching "aba" against
119 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
120 </P>
121 <P>
122 12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
123 names is not as general as Perl's. This is a consequence of the fact the PCRE
124 works internally just with numbers, using an external table to translate
125 between numbers and names. In particular, a pattern such as (?|(?&#60;a&#62;A)|(?&#60;b)B),
126 where the two capturing parentheses have the same number but different names,
127 is not supported, and causes an error at compile time. If it were allowed, it
128 would not be possible to distinguish which parentheses matched, because both
129 names map to capturing subpattern number 1. To avoid this confusing situation,
130 an error is given at compile time.
131 </P>
132 <P>
133 13. Perl recognizes comments in some places that PCRE does not, for example,
134 between the ( and ? at the start of a subpattern. If the /x modifier is set,
135 Perl allows white space between ( and ? but PCRE never does, even if the
136 PCRE_EXTENDED option is set.
137 </P>
138 <P>
139 14. PCRE provides some extensions to the Perl regular expression facilities.
140 Perl 5.10 includes new features that are not in earlier versions of Perl, some
141 of which (such as named parentheses) have been in PCRE for some time. This list
142 is with respect to Perl 5.10:
143 <br>
144 <br>
145 (a) Although lookbehind assertions in PCRE must match fixed length strings,
146 each alternative branch of a lookbehind assertion can match a different length
147 of string. Perl requires them all to have the same length.
148 <br>
149 <br>
150 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
151 meta-character matches only at the very end of the string.
152 <br>
153 <br>
154 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
155 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
156 (Perl can be made to issue a warning.)
157 <br>
158 <br>
159 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
160 inverted, that is, by default they are not greedy, but if followed by a
161 question mark they are.
162 <br>
163 <br>
164 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
165 only at the first matching position in the subject string.
166 <br>
167 <br>
169 PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents.
170 <br>
171 <br>
172 (g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
173 by the PCRE_BSR_ANYCRLF option.
174 <br>
175 <br>
176 (h) The callout facility is PCRE-specific.
177 <br>
178 <br>
179 (i) The partial matching facility is PCRE-specific.
180 <br>
181 <br>
182 (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
183 different hosts that have the other endianness. However, this does not apply to
184 optimized data created by the just-in-time compiler.
185 <br>
186 <br>
187 (k) The alternative matching functions (<b>pcre_dfa_exec()</b>,
188 <b>pcre16_dfa_exec()</b> and <b>pcre32_dfa_exec()</b>,) match in a different way
189 and are not Perl-compatible.
190 <br>
191 <br>
192 (l) PCRE recognizes some special sequences such as (*CR) at the start of
193 a pattern that set overall options that cannot be changed within the pattern.
194 </P>
195 <br><b>
197 </b><br>
198 <P>
199 Philip Hazel
200 <br>
201 University Computing Service
202 <br>
203 Cambridge CB2 3QH, England.
204 <br>
205 </P>
206 <br><b>
208 </b><br>
209 <P>
210 Last updated: 25 August 2012
211 <br>
212 Copyright &copy; 1997-2012 University of Cambridge.
213 <br>
214 <p>
215 Return to the <a href="index.html">PCRE index page</a>.
216 </p>


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