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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 byte code, so that
28 most simple patterns do not use much memory. However, there is one case where
29 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. When PCRE is compiled with its default internal
52 pointer size of two bytes, the size limit on a compiled pattern is 64K, and
53 this is reached with the above pattern if the outer repetition is increased
54 from 3 to 4. PCRE can be compiled to use larger internal pointers and thus
55 handle larger compiled patterns, but it is better to try to rewrite your
56 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> is used for matching, certain kinds of pattern can cause
81 it to use large amounts of the process stack. In some environments the default
82 process stack is quite small, and if it runs out the result is often SIGSEGV.
83 This issue is probably the most frequently raised problem with PCRE. Rewriting
84 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 scan a structure that contains data for over fifteen
103 thousand characters whenever it needs a character's property. If you can find
104 an alternative pattern that does not use character properties, it will probably
105 be faster.
106 </P>
107 <P>
108 When a pattern begins with .* not in parentheses, or in parentheses that are
109 not the subject of a backreference, and the PCRE_DOTALL option is set, the
110 pattern is implicitly anchored by PCRE, since it can match only at the start of
111 a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
112 optimization, because the . metacharacter does not then match a newline, and if
113 the subject string contains newlines, the pattern may match from the character
114 immediately following one of them instead of from the very start. For example,
115 the pattern
116 <pre>
117 .*second
118 </pre>
119 matches the subject "first\nand second" (where \n stands for a newline
120 character), with the match starting at the seventh character. In order to do
121 this, PCRE has to retry the match starting after every newline in the subject.
122 </P>
123 <P>
124 If you are using such a pattern with subject strings that do not contain
125 newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
126 the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE
127 from having to scan along the subject looking for a newline to restart at.
128 </P>
129 <P>
130 Beware of patterns that contain nested indefinite repeats. These can take a
131 long time to run when applied to a string that does not match. Consider the
132 pattern fragment
133 <pre>
134 ^(a+)*
135 </pre>
136 This can match "aaaa" in 16 different ways, and this number increases very
137 rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
138 times, and for each of those cases other than 0 or 4, the + repeats can match
139 different numbers of times.) When the remainder of the pattern is such that the
140 entire match is going to fail, PCRE has in principle to try every possible
141 variation, and this can take an extremely long time, even for relatively short
142 strings.
143 </P>
144 <P>
145 An optimization catches some of the more simple cases such as
146 <pre>
147 (a+)*b
148 </pre>
149 where a literal character follows. Before embarking on the standard matching
150 procedure, PCRE checks that there is a "b" later in the subject string, and if
151 there is not, it fails the match immediately. However, when there is no
152 following literal this optimization cannot be used. You can see the difference
153 by comparing the behaviour of
154 <pre>
155 (a+)*\d
156 </pre>
157 with the pattern above. The former gives a failure almost instantly when
158 applied to a whole line of "a" characters, whereas the latter takes an
159 appreciable time with strings longer than about 20 characters.
160 </P>
161 <P>
162 In many cases, the solution to this kind of performance issue is to use an
163 atomic group or a possessive quantifier.
164 </P>
165 <br><b>
167 </b><br>
168 <P>
169 Philip Hazel
170 <br>
171 University Computing Service
172 <br>
173 Cambridge CB2 3QH, England.
174 <br>
175 </P>
176 <br><b>
178 </b><br>
179 <P>
180 Last updated: 07 March 2010
181 <br>
182 Copyright &copy; 1997-2010 University of Cambridge.
183 <br>
184 <p>
185 Return to the <a href="index.html">PCRE index page</a>.
186 </p>


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