196 lines
7.6 KiB
HTML
196 lines
7.6 KiB
HTML
<html>
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<head>
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<title>pcreperform specification</title>
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</head>
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
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<h1>pcreperform man page</h1>
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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
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<p>
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This page is part of the PCRE HTML documentation. It was generated automatically
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from the original man page. If there is any nonsense in it, please consult the
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man page, in case the conversion went wrong.
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<br>
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<br><b>
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PCRE PERFORMANCE
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</b><br>
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<P>
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Two aspects of performance are discussed below: memory usage and processing
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time. The way you express your pattern as a regular expression can affect both
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of them.
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</P>
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<br><b>
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COMPILED PATTERN MEMORY USAGE
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</b><br>
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<P>
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Patterns are compiled by PCRE into a reasonably efficient interpretive code, so
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that most simple patterns do not use much memory. However, there is one case
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where the memory usage of a compiled pattern can be unexpectedly large. If a
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parenthesized subpattern has a quantifier with a minimum greater than 1 and/or
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a limited maximum, the whole subpattern is repeated in the compiled code. For
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example, the pattern
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<pre>
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(abc|def){2,4}
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</pre>
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is compiled as if it were
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<pre>
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(abc|def)(abc|def)((abc|def)(abc|def)?)?
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</pre>
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(Technical aside: It is done this way so that backtrack points within each of
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the repetitions can be independently maintained.)
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</P>
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<P>
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For regular expressions whose quantifiers use only small numbers, this is not
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usually a problem. However, if the numbers are large, and particularly if such
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repetitions are nested, the memory usage can become an embarrassment. For
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example, the very simple pattern
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<pre>
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((ab){1,1000}c){1,3}
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</pre>
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uses 51K bytes when compiled using the 8-bit library. When PCRE is compiled
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with its default internal pointer size of two bytes, the size limit on a
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compiled pattern is 64K data units, and this is reached with the above pattern
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if the outer repetition is increased from 3 to 4. PCRE can be compiled to use
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larger internal pointers and thus handle larger compiled patterns, but it is
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better to try to rewrite your pattern to use less memory if you can.
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</P>
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<P>
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One way of reducing the memory usage for such patterns is to make use of PCRE's
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<a href="pcrepattern.html#subpatternsassubroutines">"subroutine"</a>
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facility. Re-writing the above pattern as
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<pre>
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((ab)(?2){0,999}c)(?1){0,2}
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</pre>
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reduces the memory requirements to 18K, and indeed it remains under 20K even
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with the outer repetition increased to 100. However, this pattern is not
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exactly equivalent, because the "subroutine" calls are treated as
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<a href="pcrepattern.html#atomicgroup">atomic groups</a>
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into which there can be no backtracking if there is a subsequent matching
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failure. Therefore, PCRE cannot do this kind of rewriting automatically.
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Furthermore, there is a noticeable loss of speed when executing the modified
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pattern. Nevertheless, if the atomic grouping is not a problem and the loss of
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speed is acceptable, this kind of rewriting will allow you to process patterns
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that PCRE cannot otherwise handle.
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</P>
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<br><b>
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STACK USAGE AT RUN TIME
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</b><br>
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<P>
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When <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b> is used for matching, certain
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kinds of pattern can cause it to use large amounts of the process stack. In
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some environments the default process stack is quite small, and if it runs out
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the result is often SIGSEGV. This issue is probably the most frequently raised
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problem with PCRE. Rewriting your pattern can often help. The
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<a href="pcrestack.html"><b>pcrestack</b></a>
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documentation discusses this issue in detail.
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</P>
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<br><b>
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PROCESSING TIME
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</b><br>
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<P>
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Certain items in regular expression patterns are processed more efficiently
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than others. It is more efficient to use a character class like [aeiou] than a
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set of single-character alternatives such as (a|e|i|o|u). In general, the
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simplest construction that provides the required behaviour is usually the most
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efficient. Jeffrey Friedl's book contains a lot of useful general discussion
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about optimizing regular expressions for efficient performance. This document
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contains a few observations about PCRE.
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</P>
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<P>
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Using Unicode character properties (the \p, \P, and \X escapes) is slow,
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because PCRE has to use a multi-stage table lookup whenever it needs a
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character's property. If you can find an alternative pattern that does not use
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character properties, it will probably be faster.
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</P>
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<P>
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By default, the escape sequences \b, \d, \s, and \w, and the POSIX
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character classes such as [:alpha:] do not use Unicode properties, partly for
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backwards compatibility, and partly for performance reasons. However, you can
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set PCRE_UCP if you want Unicode character properties to be used. This can
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double the matching time for items such as \d, when matched with
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a traditional matching function; the performance loss is less with
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a DFA matching function, and in both cases there is not much difference for
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\b.
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</P>
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<P>
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When a pattern begins with .* not in parentheses, or in parentheses that are
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not the subject of a backreference, and the PCRE_DOTALL option is set, the
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pattern is implicitly anchored by PCRE, since it can match only at the start of
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a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
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optimization, because the . metacharacter does not then match a newline, and if
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the subject string contains newlines, the pattern may match from the character
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immediately following one of them instead of from the very start. For example,
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the pattern
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<pre>
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.*second
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</pre>
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matches the subject "first\nand second" (where \n stands for a newline
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character), with the match starting at the seventh character. In order to do
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this, PCRE has to retry the match starting after every newline in the subject.
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</P>
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<P>
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If you are using such a pattern with subject strings that do not contain
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newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
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the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE
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from having to scan along the subject looking for a newline to restart at.
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</P>
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<P>
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Beware of patterns that contain nested indefinite repeats. These can take a
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long time to run when applied to a string that does not match. Consider the
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pattern fragment
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<pre>
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^(a+)*
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</pre>
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This can match "aaaa" in 16 different ways, and this number increases very
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rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
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times, and for each of those cases other than 0 or 4, the + repeats can match
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different numbers of times.) When the remainder of the pattern is such that the
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entire match is going to fail, PCRE has in principle to try every possible
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variation, and this can take an extremely long time, even for relatively short
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strings.
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</P>
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<P>
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An optimization catches some of the more simple cases such as
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<pre>
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(a+)*b
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</pre>
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where a literal character follows. Before embarking on the standard matching
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procedure, PCRE checks that there is a "b" later in the subject string, and if
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there is not, it fails the match immediately. However, when there is no
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following literal this optimization cannot be used. You can see the difference
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by comparing the behaviour of
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<pre>
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(a+)*\d
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</pre>
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with the pattern above. The former gives a failure almost instantly when
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applied to a whole line of "a" characters, whereas the latter takes an
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appreciable time with strings longer than about 20 characters.
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</P>
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<P>
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In many cases, the solution to this kind of performance issue is to use an
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atomic group or a possessive quantifier.
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</P>
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<br><b>
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AUTHOR
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</b><br>
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<P>
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Philip Hazel
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<br>
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University Computing Service
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<br>
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Cambridge CB2 3QH, England.
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<br>
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</P>
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<br><b>
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REVISION
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</b><br>
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<P>
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Last updated: 25 August 2012
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<br>
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Copyright © 1997-2012 University of Cambridge.
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<br>
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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
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