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PCRE(3)


NAME

  PCRE - Perl-compatible regular expressions


PCRE PERFORMANCE

  Certain items that may appear in regular expression patterns are more
  efficient than others. It is more efficient to use a character class like
  [aeiou] than a set of 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 discussion about
  optimizing regular expressions for efficient performance.

  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 ^.* 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 33 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, 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.

  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.


Last updated: 03 February 2003

Copyright (c) 1997-2003 University of Cambridge.

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