Provided by: libpcre3-dev_8.39-9_amd64 bug


       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

       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.


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


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