NAME

       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 PERFORMANCE

       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.

COMPILED PATTERN MEMORY USAGE

       Patterns are compiled by PCRE2 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

         (abc|def){2,4}

       is compiled as if it were

         (abc|def)(abc|def)((abc|def)(abc|def)?)?

       (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

         ((ab){1,1000}c){1,3}

       uses 51K bytes when compiled using the 8-bit library. When PCRE2 is compiled with  its  default  internal
       pointer size of two bytes, the size limit on a compiled pattern is 64K code units in the 8-bit and 16-bit
       libraries,  and  this is reached with the above pattern if the outer repetition is increased from 3 to 4.
       PCRE2 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 PCRE2's  "subroutine"  facility.
       Re-writing the above pattern as

         ((ab)(?2){0,999}c)(?1){0,2}

       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, PCRE2 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 PCRE2 cannot otherwise handle.

STACK USAGE AT RUN TIME

       When pcre2_match() 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. Rewriting your pattern can often help. The  pcre2stack  documentation  discusses
       this issue in detail.

PROCESSING TIME

       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 PCRE2.

       Using  Unicode  character  properties  (the  \p,  \P, and \X escapes) is slow, because PCRE2 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 the PCRE2_UCP option or start the pattern with (*UCP) if you want Unicode character
       properties to be used. This can double the matching  time  for  items  such  as  \d,  when  matched  with
       pcre2_match();  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 atomic parentheses, nor in parentheses that are  the  subject  of  a
       backreference,  and the PCRE2_DOTALL option is set, the pattern is implicitly anchored by PCRE2, since it
       can match only at the start of a subject string. If the pattern has  multiple  top-level  branches,  they
       must  all  be  anchorable. The optimization can be disabled by the PCRE2_NO_DOTSTAR_ANCHOR option, and is
       automatically disabled if the pattern contains (*PRUNE) or (*SKIP).

       If PCRE2_DOTALL is not set, PCRE2 cannot make this optimization, because the dot 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, PCRE2 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 PCRE2_DOTALL, or starting the pattern  with  ^.*  or  ^.*?  to  indicate  explicit
       anchoring. That saves PCRE2 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 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, PCRE2 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

         (a+)*b

       where a literal character follows. Before embarking on the standard matching procedure, PCRE2 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.

       In  many  cases, the solution to this kind of performance issue is to use an atomic group or a possessive
       quantifier.

AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.

REVISION

       Last updated: 02 January 2015
       Copyright (c) 1997-2015 University of Cambridge.

PCRE2 10.00                                      02 January 2015                                 PCRE2PERFORM(3)