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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 memory usage can be unexpectedly large. When a parenthesized subpattern has a
30 quantifier with a minimum greater than 1 and/or a limited maximum, the whole
31 subpattern is repeated in the compiled code. For example, the pattern
32 <pre>
33 (abc|def){2,4}
34 </pre>
35 is compiled as if it were
36 <pre>
37 (abc|def)(abc|def)((abc|def)(abc|def)?)?
38 </pre>
39 (Technical aside: It is done this way so that backtrack points within each of
40 the repetitions can be independently maintained.)
41 </P>
42 <P>
43 For regular expressions whose quantifiers use only small numbers, this is not
44 usually a problem. However, if the numbers are large, and particularly if such
45 repetitions are nested, the memory usage can become an embarrassment. For
46 example, the very simple pattern
47 <pre>
48 ((ab){1,1000}c){1,3}
49 </pre>
50 uses 51K bytes when compiled. When PCRE is compiled with its default internal
51 pointer size of two bytes, the size limit on a compiled pattern is 64K, and
52 this is reached with the above pattern if the outer repetition is increased
53 from 3 to 4. PCRE can be compiled to use larger internal pointers and thus
54 handle larger compiled patterns, but it is better to try to rewrite your
55 pattern to use less memory if you can.
56 </P>
57 <P>
58 One way of reducing the memory usage for such patterns is to make use of PCRE's
59 <a href="pcrepattern.html#subpatternsassubroutines">"subroutine"</a>
60 facility. Re-writing the above pattern as
61 <pre>
62 ((ab)(?2){0,999}c)(?1){0,2}
63 </pre>
64 reduces the memory requirements to 18K, and indeed it remains under 20K even
65 with the outer repetition increased to 100. However, this pattern is not
66 exactly equivalent, because the "subroutine" calls are treated as
67 <a href="pcrepattern.html#atomicgroup">atomic groups</a>
68 into which there can be no backtracking if there is a subsequent matching
69 failure. Therefore, PCRE cannot do this kind of rewriting automatically.
70 Furthermore, there is a noticeable loss of speed when executing the modified
71 pattern. Nevertheless, if the atomic grouping is not a problem and the loss of
72 speed is acceptable, this kind of rewriting will allow you to process patterns
73 that PCRE cannot otherwise handle.
74 </P>
75 <br><b>
77 </b><br>
78 <P>
79 Certain items in regular expression patterns are processed more efficiently
80 than others. It is more efficient to use a character class like [aeiou] than a
81 set of single-character alternatives such as (a|e|i|o|u). In general, the
82 simplest construction that provides the required behaviour is usually the most
83 efficient. Jeffrey Friedl's book contains a lot of useful general discussion
84 about optimizing regular expressions for efficient performance. This document
85 contains a few observations about PCRE.
86 </P>
87 <P>
88 Using Unicode character properties (the \p, \P, and \X escapes) is slow,
89 because PCRE has to scan a structure that contains data for over fifteen
90 thousand characters whenever it needs a character's property. If you can find
91 an alternative pattern that does not use character properties, it will probably
92 be faster.
93 </P>
94 <P>
95 When a pattern begins with .* not in parentheses, or in parentheses that are
96 not the subject of a backreference, and the PCRE_DOTALL option is set, the
97 pattern is implicitly anchored by PCRE, since it can match only at the start of
98 a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
99 optimization, because the . metacharacter does not then match a newline, and if
100 the subject string contains newlines, the pattern may match from the character
101 immediately following one of them instead of from the very start. For example,
102 the pattern
103 <pre>
104 .*second
105 </pre>
106 matches the subject "first\nand second" (where \n stands for a newline
107 character), with the match starting at the seventh character. In order to do
108 this, PCRE has to retry the match starting after every newline in the subject.
109 </P>
110 <P>
111 If you are using such a pattern with subject strings that do not contain
112 newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
113 the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE
114 from having to scan along the subject looking for a newline to restart at.
115 </P>
116 <P>
117 Beware of patterns that contain nested indefinite repeats. These can take a
118 long time to run when applied to a string that does not match. Consider the
119 pattern fragment
120 <pre>
121 ^(a+)*
122 </pre>
123 This can match "aaaa" in 16 different ways, and this number increases very
124 rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
125 times, and for each of those cases other than 0 or 4, the + repeats can match
126 different numbers of times.) When the remainder of the pattern is such that the
127 entire match is going to fail, PCRE has in principle to try every possible
128 variation, and this can take an extremely long time, even for relatively short
129 strings.
130 </P>
131 <P>
132 An optimization catches some of the more simple cases such as
133 <pre>
134 (a+)*b
135 </pre>
136 where a literal character follows. Before embarking on the standard matching
137 procedure, PCRE checks that there is a "b" later in the subject string, and if
138 there is not, it fails the match immediately. However, when there is no
139 following literal this optimization cannot be used. You can see the difference
140 by comparing the behaviour of
141 <pre>
142 (a+)*\d
143 </pre>
144 with the pattern above. The former gives a failure almost instantly when
145 applied to a whole line of "a" characters, whereas the latter takes an
146 appreciable time with strings longer than about 20 characters.
147 </P>
148 <P>
149 In many cases, the solution to this kind of performance issue is to use an
150 atomic group or a possessive quantifier.
151 </P>
152 <br><b>
154 </b><br>
155 <P>
156 Philip Hazel
157 <br>
158 University Computing Service
159 <br>
160 Cambridge CB2 3QH, England.
161 <br>
162 </P>
163 <br><b>
165 </b><br>
166 <P>
167 Last updated: 06 March 2007
168 <br>
169 Copyright &copy; 1997-2007 University of Cambridge.
170 <br>
171 <p>
172 Return to the <a href="index.html">PCRE index page</a>.
173 </p>


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