PCREPERFORM(3)            OpenBSD Programmer's Manual           PCREPERFORM(3)



NAME
       PCRE - Perl-compatible regular expressions

PCRE PERFORMANCE

       Two aspects of performance are discussed below: memory usage and
       processing time. The way you express your pattern as a regular
       expression can affect both of them.

COMPILED PATTERN MEMORY USAGE

       Patterns are compiled by PCRE into a reasonably efficient interpretive
       code, so that most simple patterns do not use much memory. However,
       there is one case where the memory usage of a compiled pattern can be
       unexpectedly large. If a parenthesized subpattern has a quantifier with
       a minimum greater than 1 and/or a limited maximum, the whole subpattern
       is repeated in the compiled code. For example, the pattern

         (abc|def){2,4}

       is compiled as if it were

         (abc|def)(abc|def)((abc|def)(abc|def)?)?

       (Technical aside: It is done this way so that backtrack points within
       each of the repetitions can be independently maintained.)

       For regular expressions whose quantifiers use only small numbers, this
       is not usually a problem. However, if the numbers are large, and
       particularly if such repetitions are nested, the memory usage can
       become an embarrassment. For example, the very simple pattern

         ((ab){1,1000}c){1,3}

       uses 51K bytes when compiled using the 8-bit library. When PCRE is
       compiled with its default internal pointer size of two bytes, the size
       limit on a compiled pattern is 64K data units, and this is reached with
       the above pattern if the outer repetition is increased from 3 to 4.
       PCRE can be compiled to use larger internal pointers and thus handle
       larger compiled patterns, but it is better to try to rewrite your
       pattern to use less memory if you can.

       One way of reducing the memory usage for such patterns is to make use
       of PCRE's "subroutine" facility. Re-writing the above pattern as

         ((ab)(?2){0,999}c)(?1){0,2}

       reduces the memory requirements to 18K, and indeed it remains under 20K
       even with the outer repetition increased to 100. However, this pattern
       is not exactly equivalent, because the "subroutine" calls are treated
       as atomic groups into which there can be no backtracking if there is a
       subsequent matching failure. Therefore, PCRE cannot do this kind of
       rewriting automatically.  Furthermore, there is a noticeable loss of
       speed when executing the modified pattern. Nevertheless, if the atomic
       grouping is not a problem and the loss of speed is acceptable, this
       kind of rewriting will allow you to process patterns that PCRE cannot
       otherwise handle.

STACK USAGE AT RUN TIME

       When pcre_exec() or pcre16_exec() is used for matching, certain kinds
       of pattern can cause it to use large amounts of the process stack. In
       some environments the default process stack is quite small, and if it
       runs out the result is often SIGSEGV. This issue is probably the most
       frequently raised problem with PCRE. Rewriting your pattern can often
       help. The pcrestack documentation discusses this issue in detail.

PROCESSING TIME

       Certain items in regular expression patterns are processed more
       efficiently than others. It is more efficient to use a character class
       like [aeiou] than a set of single-character alternatives such as
       (a|e|i|o|u). In general, the simplest construction that provides the
       required behaviour is usually the most efficient. Jeffrey Friedl's book
       contains a lot of useful general discussion about optimizing regular
       expressions for efficient performance. This document contains a few
       observations about PCRE.

       Using Unicode character properties (the \p, \P, and \X escapes) is
       slow, because PCRE has to scan a structure that contains data for over
       fifteen thousand characters whenever it needs a character's property.
       If you can find an alternative pattern that does not use character
       properties, it will probably be faster.

       By default, the escape sequences \b, \d, \s, and \w, and the POSIX
       character classes such as [:alpha:] do not use Unicode properties,
       partly for backwards compatibility, and partly for performance reasons.
       However, you can set PCRE_UCP if you want Unicode character properties
       to be used. This can double the matching time for items such as \d,
       when matched with a traditional matching function; the performance loss
       is less with a DFA matching function, and in both cases there is not
       much difference for \b.

       When a pattern begins with .* not in parentheses, or in parentheses
       that are not the subject of a backreference, and the PCRE_DOTALL option
       is set, the pattern is implicitly anchored by PCRE, since it can match
       only at the start of a subject string. However, if PCRE_DOTALL is not
       set, PCRE cannot make this optimization, because the . metacharacter
       does not then match a newline, and if the subject string contains
       newlines, the pattern may match from the character immediately
       following one of them instead of from the very start. For example, the
       pattern

         .*second

       matches the subject "first\nand second" (where \n stands for a newline
       character), with the match starting at the seventh character. In order
       to do this, PCRE has to retry the match starting after every newline in
       the subject.

       If you are using such a pattern with subject strings that do not
       contain newlines, the best performance is obtained by setting
       PCRE_DOTALL, or starting the pattern with ^.* or ^.*? to indicate
       explicit anchoring. That saves PCRE from having to scan along the
       subject looking for a newline to restart at.

       Beware of patterns that contain nested indefinite repeats. These can
       take a long time to run when applied to a string that does not match.
       Consider the pattern fragment

         ^(a+)*

       This can match "aaaa" in 16 different ways, and this number increases
       very rapidly as the string gets longer. (The * repeat can match 0, 1,
       2, 3, or 4 times, and for each of those cases other than 0 or 4, the +
       repeats can match different numbers of times.) When the remainder of
       the pattern is such that the entire match is going to fail, PCRE has in
       principle to try every possible variation, and this can take an
       extremely long time, even for relatively short strings.

       An optimization catches some of the more simple cases such as

         (a+)*b

       where a literal character follows. Before embarking on the standard
       matching procedure, PCRE checks that there is a "b" later in the
       subject string, and if there is not, it fails the match immediately.
       However, when there is no following literal this optimization cannot be
       used. You can see the difference by comparing the behaviour of

         (a+)*\d

       with the pattern above. The former gives a failure almost instantly
       when applied to a whole line of "a" characters, whereas the latter
       takes an appreciable time with strings longer than about 20 characters.

       In many cases, the solution to this kind of performance issue is to use
       an atomic group or a possessive quantifier.

AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

       Last updated: 09 January 2012
       Copyright (c) 1997-2012 University of Cambridge.



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