Annotation of embedaddon/pcre/doc/html/pcreperform.html, revision 1.1
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! 2: <head>
! 3: <title>pcreperform specification</title>
! 4: </head>
! 5: <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
! 6: <h1>pcreperform man page</h1>
! 7: <p>
! 8: Return to the <a href="index.html">PCRE index page</a>.
! 9: </p>
! 10: <p>
! 11: This page is part of the PCRE HTML documentation. It was generated automatically
! 12: from the original man page. If there is any nonsense in it, please consult the
! 13: man page, in case the conversion went wrong.
! 14: <br>
! 15: <br><b>
! 16: PCRE PERFORMANCE
! 17: </b><br>
! 18: <P>
! 19: Two aspects of performance are discussed below: memory usage and processing
! 20: time. The way you express your pattern as a regular expression can affect both
! 21: of them.
! 22: </P>
! 23: <br><b>
! 24: COMPILED PATTERN MEMORY USAGE
! 25: </b><br>
! 26: <P>
! 27: Patterns are compiled by PCRE into a reasonably efficient byte code, so that
! 28: most simple patterns do not use much memory. However, there is one case where
! 29: the memory usage of a compiled pattern can be unexpectedly large. If a
! 30: parenthesized subpattern has a quantifier with a minimum greater than 1 and/or
! 31: a limited maximum, the whole subpattern is repeated in the compiled code. For
! 32: example, the pattern
! 33: <pre>
! 34: (abc|def){2,4}
! 35: </pre>
! 36: is compiled as if it were
! 37: <pre>
! 38: (abc|def)(abc|def)((abc|def)(abc|def)?)?
! 39: </pre>
! 40: (Technical aside: It is done this way so that backtrack points within each of
! 41: the repetitions can be independently maintained.)
! 42: </P>
! 43: <P>
! 44: For regular expressions whose quantifiers use only small numbers, this is not
! 45: usually a problem. However, if the numbers are large, and particularly if such
! 46: repetitions are nested, the memory usage can become an embarrassment. For
! 47: example, the very simple pattern
! 48: <pre>
! 49: ((ab){1,1000}c){1,3}
! 50: </pre>
! 51: uses 51K bytes when compiled. When PCRE is compiled with its default internal
! 52: pointer size of two bytes, the size limit on a compiled pattern is 64K, and
! 53: this is reached with the above pattern if the outer repetition is increased
! 54: from 3 to 4. PCRE can be compiled to use larger internal pointers and thus
! 55: handle larger compiled patterns, but it is better to try to rewrite your
! 56: pattern to use less memory if you can.
! 57: </P>
! 58: <P>
! 59: One way of reducing the memory usage for such patterns is to make use of PCRE's
! 60: <a href="pcrepattern.html#subpatternsassubroutines">"subroutine"</a>
! 61: facility. Re-writing the above pattern as
! 62: <pre>
! 63: ((ab)(?2){0,999}c)(?1){0,2}
! 64: </pre>
! 65: reduces the memory requirements to 18K, and indeed it remains under 20K even
! 66: with the outer repetition increased to 100. However, this pattern is not
! 67: exactly equivalent, because the "subroutine" calls are treated as
! 68: <a href="pcrepattern.html#atomicgroup">atomic groups</a>
! 69: into which there can be no backtracking if there is a subsequent matching
! 70: failure. Therefore, PCRE cannot do this kind of rewriting automatically.
! 71: Furthermore, there is a noticeable loss of speed when executing the modified
! 72: pattern. Nevertheless, if the atomic grouping is not a problem and the loss of
! 73: speed is acceptable, this kind of rewriting will allow you to process patterns
! 74: that PCRE cannot otherwise handle.
! 75: </P>
! 76: <br><b>
! 77: STACK USAGE AT RUN TIME
! 78: </b><br>
! 79: <P>
! 80: When <b>pcre_exec()</b> is used for matching, certain kinds of pattern can cause
! 81: it to use large amounts of the process stack. In some environments the default
! 82: process stack is quite small, and if it runs out the result is often SIGSEGV.
! 83: This issue is probably the most frequently raised problem with PCRE. Rewriting
! 84: your pattern can often help. The
! 85: <a href="pcrestack.html"><b>pcrestack</b></a>
! 86: documentation discusses this issue in detail.
! 87: </P>
! 88: <br><b>
! 89: PROCESSING TIME
! 90: </b><br>
! 91: <P>
! 92: Certain items in regular expression patterns are processed more efficiently
! 93: than others. It is more efficient to use a character class like [aeiou] than a
! 94: set of single-character alternatives such as (a|e|i|o|u). In general, the
! 95: simplest construction that provides the required behaviour is usually the most
! 96: efficient. Jeffrey Friedl's book contains a lot of useful general discussion
! 97: about optimizing regular expressions for efficient performance. This document
! 98: contains a few observations about PCRE.
! 99: </P>
! 100: <P>
! 101: Using Unicode character properties (the \p, \P, and \X escapes) is slow,
! 102: because PCRE has to scan a structure that contains data for over fifteen
! 103: thousand characters whenever it needs a character's property. If you can find
! 104: an alternative pattern that does not use character properties, it will probably
! 105: be faster.
! 106: </P>
! 107: <P>
! 108: By default, the escape sequences \b, \d, \s, and \w, and the POSIX
! 109: character classes such as [:alpha:] do not use Unicode properties, partly for
! 110: backwards compatibility, and partly for performance reasons. However, you can
! 111: set PCRE_UCP if you want Unicode character properties to be used. This can
! 112: double the matching time for items such as \d, when matched with
! 113: <b>pcre_exec()</b>; the performance loss is less with <b>pcre_dfa_exec()</b>, and
! 114: in both cases there is not much difference for \b.
! 115: </P>
! 116: <P>
! 117: When a pattern begins with .* not in parentheses, or in parentheses that are
! 118: not the subject of a backreference, and the PCRE_DOTALL option is set, the
! 119: pattern is implicitly anchored by PCRE, since it can match only at the start of
! 120: a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
! 121: optimization, because the . metacharacter does not then match a newline, and if
! 122: the subject string contains newlines, the pattern may match from the character
! 123: immediately following one of them instead of from the very start. For example,
! 124: the pattern
! 125: <pre>
! 126: .*second
! 127: </pre>
! 128: matches the subject "first\nand second" (where \n stands for a newline
! 129: character), with the match starting at the seventh character. In order to do
! 130: this, PCRE has to retry the match starting after every newline in the subject.
! 131: </P>
! 132: <P>
! 133: If you are using such a pattern with subject strings that do not contain
! 134: newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
! 135: the pattern with ^.* or ^.*? to indicate explicit anchoring. That saves PCRE
! 136: from having to scan along the subject looking for a newline to restart at.
! 137: </P>
! 138: <P>
! 139: Beware of patterns that contain nested indefinite repeats. These can take a
! 140: long time to run when applied to a string that does not match. Consider the
! 141: pattern fragment
! 142: <pre>
! 143: ^(a+)*
! 144: </pre>
! 145: This can match "aaaa" in 16 different ways, and this number increases very
! 146: rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
! 147: times, and for each of those cases other than 0 or 4, the + repeats can match
! 148: different numbers of times.) When the remainder of the pattern is such that the
! 149: entire match is going to fail, PCRE has in principle to try every possible
! 150: variation, and this can take an extremely long time, even for relatively short
! 151: strings.
! 152: </P>
! 153: <P>
! 154: An optimization catches some of the more simple cases such as
! 155: <pre>
! 156: (a+)*b
! 157: </pre>
! 158: where a literal character follows. Before embarking on the standard matching
! 159: procedure, PCRE checks that there is a "b" later in the subject string, and if
! 160: there is not, it fails the match immediately. However, when there is no
! 161: following literal this optimization cannot be used. You can see the difference
! 162: by comparing the behaviour of
! 163: <pre>
! 164: (a+)*\d
! 165: </pre>
! 166: with the pattern above. The former gives a failure almost instantly when
! 167: applied to a whole line of "a" characters, whereas the latter takes an
! 168: appreciable time with strings longer than about 20 characters.
! 169: </P>
! 170: <P>
! 171: In many cases, the solution to this kind of performance issue is to use an
! 172: atomic group or a possessive quantifier.
! 173: </P>
! 174: <br><b>
! 175: AUTHOR
! 176: </b><br>
! 177: <P>
! 178: Philip Hazel
! 179: <br>
! 180: University Computing Service
! 181: <br>
! 182: Cambridge CB2 3QH, England.
! 183: <br>
! 184: </P>
! 185: <br><b>
! 186: REVISION
! 187: </b><br>
! 188: <P>
! 189: Last updated: 16 May 2010
! 190: <br>
! 191: Copyright © 1997-2010 University of Cambridge.
! 192: <br>
! 193: <p>
! 194: Return to the <a href="index.html">PCRE index page</a>.
! 195: </p>
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