PCREPERFORM(3)             Library Functions Manual             PCREPERFORM(3)

       PCRE - Perl-compatible regular expressions


       Two  aspects  of performance are discussed below: memory usage and pro-
       cessing 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  par-
       ticularly  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 com-
       piled  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 pat-
       tern 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 de-


       Certain items in regular expression patterns are processed  more  effi-
       ciently 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 ob-
       servations 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  new-
       lines,  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 con-
       tain newlines, the best performance is obtained by setting PCRE_DOTALL,
       or  starting  the pattern with ^.* or ^.*? to indicate explicit anchor-
       ing. 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  ex-
       tremely 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  sub-
       ject  string, and if there is not, it fails the match immediately. How-
       ever, 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.


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


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

PCRE 8.30                       09 January 2012                 PCREPERFORM(3)
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