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Linux 2.6 - man page for pcreperform (linux section 3)


       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 byte 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  repeti-
       tions 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. When PCRE is compiled with its default internal pointer size
       of two bytes, the size limit on a compiled pattern is 64K, 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 "subrou-
       tine" 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() 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  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 prop-
       erties to be used. This can double the matching time for items such as  \d,  when  matched
       with  pcre_exec();  the	performance  loss is less with pcre_dfa_exec(), 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  sub-
       ject  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 prin-
       ciple 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 can-
       not 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.


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


       Last updated: 16 May 2010
       Copyright (c) 1997-2010 University of Cambridge.


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