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1 <html>
2 <head>
3 <title>pcreperform specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcreperform 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 Two aspects of performance are discussed below: memory usage and processing
20 time. The way you express your pattern as a regular expression can affect both
21 of them.
22 </P>
23 <br><b>
25 </b><br>
26 <P>
27 Patterns are compiled by PCRE into a reasonably efficient interpretive code, so
28 that most simple patterns do not use much memory. However, there is one case
29 where the memory usage of a compiled pattern can be unexpectedly large. If a
30 parenthesized subpattern has a quantifier with a minimum greater than 1 and/or
31 a limited maximum, the whole subpattern is repeated in the compiled code. For
32 example, the pattern
33 <pre>
34 (abc|def){2,4}
35 </pre>
36 is compiled as if it were
37 <pre>
38 (abc|def)(abc|def)((abc|def)(abc|def)?)?
39 </pre>
40 (Technical aside: It is done this way so that backtrack points within each of
41 the repetitions can be independently maintained.)
42 </P>
43 <P>
44 For regular expressions whose quantifiers use only small numbers, this is not
45 usually a problem. However, if the numbers are large, and particularly if such
46 repetitions are nested, the memory usage can become an embarrassment. For
47 example, the very simple pattern
48 <pre>
49 ((ab){1,1000}c){1,3}
50 </pre>
51 uses 51K bytes when compiled using the 8-bit library. When PCRE is compiled
52 with its default internal pointer size of two bytes, the size limit on a
53 compiled pattern is 64K data units, and this is reached with the above pattern
54 if the outer repetition is increased from 3 to 4. PCRE can be compiled to use
55 larger internal pointers and thus handle larger compiled patterns, but it is
56 better to try to rewrite your pattern to use less memory if you can.
57 </P>
58 <P>
59 One way of reducing the memory usage for such patterns is to make use of PCRE's
60 <a href="pcrepattern.html#subpatternsassubroutines">"subroutine"</a>
61 facility. Re-writing the above pattern as
62 <pre>
63 ((ab)(?2){0,999}c)(?1){0,2}
64 </pre>
65 reduces the memory requirements to 18K, and indeed it remains under 20K even
66 with the outer repetition increased to 100. However, this pattern is not
67 exactly equivalent, because the "subroutine" calls are treated as
68 <a href="pcrepattern.html#atomicgroup">atomic groups</a>
69 into which there can be no backtracking if there is a subsequent matching
70 failure. Therefore, PCRE cannot do this kind of rewriting automatically.
71 Furthermore, there is a noticeable loss of speed when executing the modified
72 pattern. Nevertheless, if the atomic grouping is not a problem and the loss of
73 speed is acceptable, this kind of rewriting will allow you to process patterns
74 that PCRE cannot otherwise handle.
75 </P>
76 <br><b>
78 </b><br>
79 <P>
80 When <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b> is used for matching, certain
81 kinds of pattern can cause it to use large amounts of the process stack. In
82 some environments the default process stack is quite small, and if it runs out
83 the result is often SIGSEGV. This issue is probably the most frequently raised
84 problem with PCRE. Rewriting your pattern can often help. The
85 <a href="pcrestack.html"><b>pcrestack</b></a>
86 documentation discusses this issue in detail.
87 </P>
88 <br><b>
90 </b><br>
91 <P>
92 Certain items in regular expression patterns are processed more efficiently
93 than others. It is more efficient to use a character class like [aeiou] than a
94 set of single-character alternatives such as (a|e|i|o|u). In general, the
95 simplest construction that provides the required behaviour is usually the most
96 efficient. Jeffrey Friedl's book contains a lot of useful general discussion
97 about optimizing regular expressions for efficient performance. This document
98 contains a few observations about PCRE.
99 </P>
100 <P>
101 Using Unicode character properties (the \p, \P, and \X escapes) is slow,
102 because PCRE has to use a multi-stage table lookup whenever it needs a
103 character's property. If you can find an alternative pattern that does not use
104 character properties, it will probably be faster.
105 </P>
106 <P>
107 By default, the escape sequences \b, \d, \s, and \w, and the POSIX
108 character classes such as [:alpha:] do not use Unicode properties, partly for
109 backwards compatibility, and partly for performance reasons. However, you can
110 set PCRE_UCP if you want Unicode character properties to be used. This can
111 double the matching time for items such as \d, when matched with
112 a traditional matching function; the performance loss is less with
113 a DFA matching function, and in both cases there is not much difference for
114 \b.
115 </P>
116 <P>
117 When a pattern begins with .* not in parentheses, or in parentheses that are
118 not the subject of a backreference, and the PCRE_DOTALL option is set, the
119 pattern is implicitly anchored by PCRE, since it can match only at the start of
120 a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
121 optimization, because the . metacharacter does not then match a newline, and if
122 the subject string contains newlines, the pattern may match from the character
123 immediately following one of them instead of from the very start. For example,
124 the pattern
125 <pre>
126 .*second
127 </pre>
128 matches the subject "first\nand second" (where \n stands for a newline
129 character), with the match starting at the seventh character. In order to do
130 this, PCRE has to retry the match starting after every newline in the subject.
131 </P>
132 <P>
133 If you are using such a pattern with subject strings that do not contain
134 newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
135 the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE
136 from having to scan along the subject looking for a newline to restart at.
137 </P>
138 <P>
139 Beware of patterns that contain nested indefinite repeats. These can take a
140 long time to run when applied to a string that does not match. Consider the
141 pattern fragment
142 <pre>
143 ^(a+)*
144 </pre>
145 This can match "aaaa" in 16 different ways, and this number increases very
146 rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
147 times, and for each of those cases other than 0 or 4, the + repeats can match
148 different numbers of times.) When the remainder of the pattern is such that the
149 entire match is going to fail, PCRE has in principle to try every possible
150 variation, and this can take an extremely long time, even for relatively short
151 strings.
152 </P>
153 <P>
154 An optimization catches some of the more simple cases such as
155 <pre>
156 (a+)*b
157 </pre>
158 where a literal character follows. Before embarking on the standard matching
159 procedure, PCRE checks that there is a "b" later in the subject string, and if
160 there is not, it fails the match immediately. However, when there is no
161 following literal this optimization cannot be used. You can see the difference
162 by comparing the behaviour of
163 <pre>
164 (a+)*\d
165 </pre>
166 with the pattern above. The former gives a failure almost instantly when
167 applied to a whole line of "a" characters, whereas the latter takes an
168 appreciable time with strings longer than about 20 characters.
169 </P>
170 <P>
171 In many cases, the solution to this kind of performance issue is to use an
172 atomic group or a possessive quantifier.
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: 25 August 2012
190 <br>
191 Copyright &copy; 1997-2012 University of Cambridge.
192 <br>
193 <p>
194 Return to the <a href="index.html">PCRE index page</a>.
195 </p>


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