pcreperform(3) — Linux manual page


PCREPERFORM(3)          Library Functions Manual          PCREPERFORM(3)

NAME         top

       PCRE - Perl-compatible regular expressions


       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.


       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


       is compiled as if it were


       (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


       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


       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.


       When pcre_exec() or pcre[16|32]_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.


       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 use a multi-stage table lookup
       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


       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


       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


       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


       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         top

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

REVISION         top

       Last updated: 25 August 2012
       Copyright (c) 1997-2012 University of Cambridge.

COLOPHON         top

       This page is part of the PCRE (Perl Compatible Regular
       Expressions) project.  Information about the project can be found
       at ⟨http://www.pcre.org/⟩.  If you have a bug report for this
       manual page, see
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PCRE 8.30                    09 January 2012              PCREPERFORM(3)