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16  PCRE PERFORMANCE  PCRE PERFORMANCE
17  </b><br>  </b><br>
18  <P>  <P>
19  Certain items that may appear in regular expression patterns are more efficient  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    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    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>
76    PROCESSING TIME
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  than others. It is more efficient to use a character class like [aeiou] than a
81  set of alternatives such as (a|e|i|o|u). In general, the simplest construction  set of single-character alternatives such as (a|e|i|o|u). In general, the
82  that provides the required behaviour is usually the most efficient. Jeffrey  simplest construction that provides the required behaviour is usually the most
83  Friedl's book contains a lot of useful general discussion about optimizing  efficient. Jeffrey Friedl's book contains a lot of useful general discussion
84  regular expressions for efficient performance. This document contains a few  about optimizing regular expressions for efficient performance. This document
85  observations about PCRE.  contains a few observations about PCRE.
86  </P>  </P>
87  <P>  <P>
88  Using Unicode character properties (the \p, \P, and \X escapes) is slow,  Using Unicode character properties (the \p, \P, and \X escapes) is slow,
# Line 58  Beware of patterns that contain nested i Line 118  Beware of patterns that contain nested i
118  long time to run when applied to a string that does not match. Consider the  long time to run when applied to a string that does not match. Consider the
119  pattern fragment  pattern fragment
120  <pre>  <pre>
121    (a+)*    ^(a+)*
122  </pre>  </pre>
123  This can match "aaaa" in 33 different ways, and this number increases very  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  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, the + repeats can match  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  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  entire match is going to fail, PCRE has in principle to try every possible
128  variation, and this can take an extremely long time.  variation, and this can take an extremely long time, even for relatively short
129    strings.
130  </P>  </P>
131  <P>  <P>
132  An optimization catches some of the more simple cases such as  An optimization catches some of the more simple cases such as
# Line 89  In many cases, the solution to this kind Line 150  In many cases, the solution to this kind
150  atomic group or a possessive quantifier.  atomic group or a possessive quantifier.
151  </P>  </P>
152  <P>  <P>
153  Last updated: 28 February 2005  Last updated: 20 September 2006
154  <br>  <br>
155  Copyright &copy; 1997-2005 University of Cambridge.  Copyright &copy; 1997-2006 University of Cambridge.
156  <p>  <p>
157  Return to the <a href="index.html">PCRE index page</a>.  Return to the <a href="index.html">PCRE index page</a>.
158  </p>  </p>

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