Annotation of embedaddon/pcre/doc/pcreperform.3, revision 1.1
1.1 ! misho 1: .TH PCREPERFORM 3
! 2: .SH NAME
! 3: PCRE - Perl-compatible regular expressions
! 4: .SH "PCRE PERFORMANCE"
! 5: .rs
! 6: .sp
! 7: Two aspects of performance are discussed below: memory usage and processing
! 8: time. The way you express your pattern as a regular expression can affect both
! 9: of them.
! 10: .
! 11: .SH "COMPILED PATTERN MEMORY USAGE"
! 12: .rs
! 13: .sp
! 14: Patterns are compiled by PCRE into a reasonably efficient byte code, so that
! 15: most simple patterns do not use much memory. However, there is one case where
! 16: the memory usage of a compiled pattern can be unexpectedly large. If a
! 17: parenthesized subpattern has a quantifier with a minimum greater than 1 and/or
! 18: a limited maximum, the whole subpattern is repeated in the compiled code. For
! 19: example, the pattern
! 20: .sp
! 21: (abc|def){2,4}
! 22: .sp
! 23: is compiled as if it were
! 24: .sp
! 25: (abc|def)(abc|def)((abc|def)(abc|def)?)?
! 26: .sp
! 27: (Technical aside: It is done this way so that backtrack points within each of
! 28: the repetitions can be independently maintained.)
! 29: .P
! 30: For regular expressions whose quantifiers use only small numbers, this is not
! 31: usually a problem. However, if the numbers are large, and particularly if such
! 32: repetitions are nested, the memory usage can become an embarrassment. For
! 33: example, the very simple pattern
! 34: .sp
! 35: ((ab){1,1000}c){1,3}
! 36: .sp
! 37: uses 51K bytes when compiled. When PCRE is compiled with its default internal
! 38: pointer size of two bytes, the size limit on a compiled pattern is 64K, and
! 39: this is reached with the above pattern if the outer repetition is increased
! 40: from 3 to 4. PCRE can be compiled to use larger internal pointers and thus
! 41: handle larger compiled patterns, but it is better to try to rewrite your
! 42: pattern to use less memory if you can.
! 43: .P
! 44: One way of reducing the memory usage for such patterns is to make use of PCRE's
! 45: .\" HTML <a href="pcrepattern.html#subpatternsassubroutines">
! 46: .\" </a>
! 47: "subroutine"
! 48: .\"
! 49: facility. Re-writing the above pattern as
! 50: .sp
! 51: ((ab)(?2){0,999}c)(?1){0,2}
! 52: .sp
! 53: reduces the memory requirements to 18K, and indeed it remains under 20K even
! 54: with the outer repetition increased to 100. However, this pattern is not
! 55: exactly equivalent, because the "subroutine" calls are treated as
! 56: .\" HTML <a href="pcrepattern.html#atomicgroup">
! 57: .\" </a>
! 58: atomic groups
! 59: .\"
! 60: into which there can be no backtracking if there is a subsequent matching
! 61: failure. Therefore, PCRE cannot do this kind of rewriting automatically.
! 62: Furthermore, there is a noticeable loss of speed when executing the modified
! 63: pattern. Nevertheless, if the atomic grouping is not a problem and the loss of
! 64: speed is acceptable, this kind of rewriting will allow you to process patterns
! 65: that PCRE cannot otherwise handle.
! 66: .
! 67: .
! 68: .SH "STACK USAGE AT RUN TIME"
! 69: .rs
! 70: .sp
! 71: When \fBpcre_exec()\fP is used for matching, certain kinds of pattern can cause
! 72: it to use large amounts of the process stack. In some environments the default
! 73: process stack is quite small, and if it runs out the result is often SIGSEGV.
! 74: This issue is probably the most frequently raised problem with PCRE. Rewriting
! 75: your pattern can often help. The
! 76: .\" HREF
! 77: \fBpcrestack\fP
! 78: .\"
! 79: documentation discusses this issue in detail.
! 80: .
! 81: .
! 82: .SH "PROCESSING TIME"
! 83: .rs
! 84: .sp
! 85: Certain items in regular expression patterns are processed more efficiently
! 86: than others. It is more efficient to use a character class like [aeiou] than a
! 87: set of single-character alternatives such as (a|e|i|o|u). In general, the
! 88: simplest construction that provides the required behaviour is usually the most
! 89: efficient. Jeffrey Friedl's book contains a lot of useful general discussion
! 90: about optimizing regular expressions for efficient performance. This document
! 91: contains a few observations about PCRE.
! 92: .P
! 93: Using Unicode character properties (the \ep, \eP, and \eX escapes) is slow,
! 94: because PCRE has to scan a structure that contains data for over fifteen
! 95: thousand characters whenever it needs a character's property. If you can find
! 96: an alternative pattern that does not use character properties, it will probably
! 97: be faster.
! 98: .P
! 99: By default, the escape sequences \eb, \ed, \es, and \ew, and the POSIX
! 100: character classes such as [:alpha:] do not use Unicode properties, partly for
! 101: backwards compatibility, and partly for performance reasons. However, you can
! 102: set PCRE_UCP if you want Unicode character properties to be used. This can
! 103: double the matching time for items such as \ed, when matched with
! 104: \fBpcre_exec()\fP; the performance loss is less with \fBpcre_dfa_exec()\fP, and
! 105: in both cases there is not much difference for \eb.
! 106: .P
! 107: When a pattern begins with .* not in parentheses, or in parentheses that are
! 108: not the subject of a backreference, and the PCRE_DOTALL option is set, the
! 109: pattern is implicitly anchored by PCRE, since it can match only at the start of
! 110: a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
! 111: optimization, because the . metacharacter does not then match a newline, and if
! 112: the subject string contains newlines, the pattern may match from the character
! 113: immediately following one of them instead of from the very start. For example,
! 114: the pattern
! 115: .sp
! 116: .*second
! 117: .sp
! 118: matches the subject "first\enand second" (where \en stands for a newline
! 119: character), with the match starting at the seventh character. In order to do
! 120: this, PCRE has to retry the match starting after every newline in the subject.
! 121: .P
! 122: If you are using such a pattern with subject strings that do not contain
! 123: newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
! 124: the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE
! 125: from having to scan along the subject looking for a newline to restart at.
! 126: .P
! 127: Beware of patterns that contain nested indefinite repeats. These can take a
! 128: long time to run when applied to a string that does not match. Consider the
! 129: pattern fragment
! 130: .sp
! 131: ^(a+)*
! 132: .sp
! 133: This can match "aaaa" in 16 different ways, and this number increases very
! 134: rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
! 135: times, and for each of those cases other than 0 or 4, the + repeats can match
! 136: different numbers of times.) When the remainder of the pattern is such that the
! 137: entire match is going to fail, PCRE has in principle to try every possible
! 138: variation, and this can take an extremely long time, even for relatively short
! 139: strings.
! 140: .P
! 141: An optimization catches some of the more simple cases such as
! 142: .sp
! 143: (a+)*b
! 144: .sp
! 145: where a literal character follows. Before embarking on the standard matching
! 146: procedure, PCRE checks that there is a "b" later in the subject string, and if
! 147: there is not, it fails the match immediately. However, when there is no
! 148: following literal this optimization cannot be used. You can see the difference
! 149: by comparing the behaviour of
! 150: .sp
! 151: (a+)*\ed
! 152: .sp
! 153: with the pattern above. The former gives a failure almost instantly when
! 154: applied to a whole line of "a" characters, whereas the latter takes an
! 155: appreciable time with strings longer than about 20 characters.
! 156: .P
! 157: In many cases, the solution to this kind of performance issue is to use an
! 158: atomic group or a possessive quantifier.
! 159: .
! 160: .
! 161: .SH AUTHOR
! 162: .rs
! 163: .sp
! 164: .nf
! 165: Philip Hazel
! 166: University Computing Service
! 167: Cambridge CB2 3QH, England.
! 168: .fi
! 169: .
! 170: .
! 171: .SH REVISION
! 172: .rs
! 173: .sp
! 174: .nf
! 175: Last updated: 16 May 2010
! 176: Copyright (c) 1997-2010 University of Cambridge.
! 177: .fi
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