PCREPERFORM(3)            Library Functions Manual            PCREPERFORM(3)

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       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 as


       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

       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.

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PCRE 8.30                      09 January 2012                PCREPERFORM(3)