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3: <title>pcreperform specification</title>
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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>
16: PCRE PERFORMANCE
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>
24: COMPILED PATTERN MEMORY USAGE
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>
77: STACK USAGE AT RUN TIME
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>
89: PROCESSING TIME
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: By default, the escape sequences \b, \d, \s, and \w, and the POSIX
109: character classes such as [:alpha:] do not use Unicode properties, partly for
110: backwards compatibility, and partly for performance reasons. However, you can
111: set PCRE_UCP if you want Unicode character properties to be used. This can
112: double the matching time for items such as \d, when matched with
113: <b>pcre_exec()</b>; the performance loss is less with <b>pcre_dfa_exec()</b>, and
114: in both cases there is not much difference for \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>
175: AUTHOR
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>
186: REVISION
187: </b><br>
188: <P>
189: Last updated: 16 May 2010
190: <br>
191: Copyright © 1997-2010 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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