pcre2perform

PCRE2PERFORM(3)            Library Functions Manual            PCRE2PERFORM(3)

NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 PERFORMANCE

       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.

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  for  storing
       the compiled version. However, there is one case where the memory usage
       of a compiled pattern can be unexpectedly  large.  If  a  parenthesized
       group  has  a quantifier with a minimum greater than 1 and/or a limited
       maximum, the whole group 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 par-
       ticularly 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  over  50KiB  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 65535 code units in the 8-bit and 16-bit
       libraries, and this is reached with the above pattern if the outer rep-
       etition  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 around 16KiB, and indeed it  remains
       under  20KiB  even with the outer repetition increased to 100. However,
       this kind of pattern is not always exactly equivalent, because any cap-
       tures  within  subroutine calls are lost when the subroutine completes.
       If this is not a problem, this kind of  rewriting  will  allow  you  to
       process  patterns that PCRE2 cannot otherwise handle. The matching per-
       formance of the two different versions of the pattern are  roughly  the
       same.  (This applies from release 10.30 - things were different in ear-
       lier releases.)

STACK AND HEAP USAGE AT RUN TIME

       From release 10.30, the interpretive (non-JIT) version of pcre2_match()
       uses  very  little system stack at run time. In earlier releases recur-
       sive function calls could use a great deal of  stack,  and  this  could
       cause  problems, but this usage has been eliminated. Backtracking posi-
       tions are now explicitly remembered in memory frames controlled by  the
       code.  An  initial  20KiB  vector  of frames is allocated on the system
       stack (enough for about 100 frames for small patterns), but if this  is
       insufficient,  heap  memory  is  used. The amount of heap memory can be
       limited; if the limit is set to zero, only the initial stack vector  is
       used.  Rewriting patterns to be time-efficient, as described below, may
       also reduce the memory requirements.

       In contrast to  pcre2_match(),  pcre2_dfa_match()  does  use  recursive
       function  calls,  but only for processing atomic groups, lookaround as-
       sertions, and recursion within the pattern. The original version of the
       code  used  to  allocate  quite large internal workspace vectors on the
       stack, which caused some problems for  some  patterns  in  environments
       with  small  stacks.  From release 10.32 the code for pcre2_dfa_match()
       has been re-factored to use heap memory  when  necessary  for  internal
       workspace  when  recursing,  though  recursive function calls are still
       used.

       The "match depth" parameter can be used to limit the depth of  function
       recursion,  and  the  "match  heap"  parameter  to limit heap memory in
       pcre2_dfa_match().

PROCESSING TIME

       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 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  func-
       tion, and in both cases there is not much difference for \b.

       When  a pattern begins with .* not in atomic parentheses, nor in paren-
       theses 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 optimiza-
       tion can be disabled by the PCRE2_NO_DOTSTAR_ANCHOR option, and is  au-
       tomatically disabled if the pattern contains (*PRUNE) or (*SKIP).

       If  PCRE2_DOTALL  is  not set, PCRE2 cannot make this optimization, be-
       cause the dot metacharacter does not then match a newline, and  if  the
       subject  string contains newlines, the pattern may match from the char-
       acter 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  con-
       tain   newlines,   the   best   performance   is  obtained  by  setting
       PCRE2_DOTALL, or starting the pattern with ^.* or ^.*? to indicate  ex-
       plicit  anchoring.  That saves PCRE2 from having to scan along the sub-
       ject 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 ex-
       tremely 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 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

         (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. This can often reduce mem-
       ory requirements as well. As another example, consider this pattern:

         ([^<]|<(?!inet))+

       It matches from wherever it starts until it encounters "<inet"  or  the
       end  of  the  data,  and is the kind of pattern that might be used when
       processing an XML file. Each iteration of the outer parentheses matches
       either  one  character that is not "<" or a "<" that is not followed by
       "inet". However, each time a parenthesis is processed,  a  backtracking
       position  is  passed,  so this formulation uses a memory frame for each
       matched character. For a long string, a lot of memory is required. Con-
       sider  now  this  rewritten  pattern,  which  matches  exactly the same
       strings:

         ([^<]++|<(?!inet))+

       This runs much faster, because sequences of characters that do not con-
       tain "<" are "swallowed" in one item inside the parentheses, and a pos-
       sessive quantifier is used to stop any backtracking into  the  runs  of
       non-"<"  characters.  This  version also uses a lot less memory because
       entry to a new set of parentheses happens only  when  a  "<"  character
       that  is  not  followed by "inet" is encountered (and we assume this is
       relatively rare).

       This example shows that one way of optimizing performance when matching
       long  subject strings is to write repeated parenthesized subpatterns to
       match more than one character whenever possible.

   SETTING RESOURCE LIMITS

       You can set limits on the amount of processing that  takes  place  when
       matching,  and  on  the amount of heap memory that is used. The default
       values of the limits are very large, and unlikely ever to operate. They
       can  be  changed  when  PCRE2  is  built, and they can also be set when
       pcre2_match() or pcre2_dfa_match() is called. For details of these  in-
       terfaces,  see  the  pcre2build  documentation and the section entitled
       "The match context" in the pcre2api documentation.

       The pcre2test test program has a modifier called  "find_limits"  which,
       if  applied  to  a  subject line, causes it to find the smallest limits
       that allow a pattern to match. This is done by repeatedly matching with
       different limits.

AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.

REVISION

       Last updated: 03 February 2019
       Copyright (c) 1997-2019 University of Cambridge.

PCRE2 10.33                    03 February 2019                PCRE2PERFORM(3)