2014-10-20 18:48:14 +02:00
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<title>pcre2stack specification</title>
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
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<h1>pcre2stack man page</h1>
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<p>
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Return to the <a href="index.html">PCRE2 index page</a>.
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</p>
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<p>
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This page is part of the PCRE2 HTML documentation. It was generated
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automatically from the original man page. If there is any nonsense in it,
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please consult the man page, in case the conversion went wrong.
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<br>
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<br><b>
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PCRE2 DISCUSSION OF STACK USAGE
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</b><br>
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<P>
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When you call <b>pcre2_match()</b>, it makes use of an internal function called
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<b>match()</b>. This calls itself recursively at branch points in the pattern,
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in order to remember the state of the match so that it can back up and try a
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different alternative after a failure. As matching proceeds deeper and deeper
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into the tree of possibilities, the recursion depth increases. The
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<b>match()</b> function is also called in other circumstances, for example,
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whenever a parenthesized sub-pattern is entered, and in certain cases of
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repetition.
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</P>
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<P>
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Not all calls of <b>match()</b> increase the recursion depth; for an item such
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as a* it may be called several times at the same level, after matching
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different numbers of a's. Furthermore, in a number of cases where the result of
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the recursive call would immediately be passed back as the result of the
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current call (a "tail recursion"), the function is just restarted instead.
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</P>
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<P>
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The above comments apply when <b>pcre2_match()</b> is run in its normal
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interpretive manner. If the compiled pattern was processed by
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<b>pcre2_jit_compile()</b>, and just-in-time compiling was successful, and the
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options passed to <b>pcre2_match()</b> were not incompatible, the matching
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process uses the JIT-compiled code instead of the <b>match()</b> function. In
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this case, the memory requirements are handled entirely differently. See the
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<a href="pcre2jit.html"><b>pcre2jit</b></a>
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documentation for details.
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</P>
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<P>
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The <b>pcre2_dfa_match()</b> function operates in a different way to
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<b>pcre2_match()</b>, and uses recursion only when there is a regular expression
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recursion or subroutine call in the pattern. This includes the processing of
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assertion and "once-only" subpatterns, which are handled like subroutine calls.
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Normally, these are never very deep, and the limit on the complexity of
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<b>pcre2_dfa_match()</b> is controlled by the amount of workspace it is given.
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However, it is possible to write patterns with runaway infinite recursions;
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such patterns will cause <b>pcre2_dfa_match()</b> to run out of stack. At
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present, there is no protection against this.
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</P>
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<P>
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The comments that follow do NOT apply to <b>pcre2_dfa_match()</b>; they are
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relevant only for <b>pcre2_match()</b> without the JIT optimization.
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</P>
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<br><b>
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Reducing <b>pcre2_match()</b>'s stack usage
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</b><br>
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<P>
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Each time that the internal <b>match()</b> function is called recursively, it
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uses memory from the process stack. For certain kinds of pattern and data, very
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large amounts of stack may be needed, despite the recognition of "tail
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recursion". You can often reduce the amount of recursion, and therefore the
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amount of stack used, by modifying the pattern that is being matched. Consider,
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for example, this pattern:
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<pre>
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([^<]|<(?!inet))+
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</pre>
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It matches from wherever it starts until it encounters "<inet" or the end of
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the data, and is the kind of pattern that might be used when processing an XML
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file. Each iteration of the outer parentheses matches either one character that
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is not "<" or a "<" that is not followed by "inet". However, each time a
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parenthesis is processed, a recursion occurs, so this formulation uses a stack
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frame for each matched character. For a long string, a lot of stack is
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required. Consider now this rewritten pattern, which matches exactly the same
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strings:
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<pre>
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([^<]++|<(?!inet))+
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</pre>
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This uses very much less stack, because runs of characters that do not contain
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"<" are "swallowed" in one item inside the parentheses. Recursion happens only
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when a "<" character that is not followed by "inet" is encountered (and we
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assume this is relatively rare). A possessive quantifier is used to stop any
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backtracking into the runs of non-"<" characters, but that is not related to
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stack usage.
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</P>
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<P>
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This example shows that one way of avoiding stack problems when matching long
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subject strings is to write repeated parenthesized subpatterns to match more
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than one character whenever possible.
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</P>
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<br><b>
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Compiling PCRE2 to use heap instead of stack for <b>pcre2_match()</b>
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</b><br>
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<P>
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In environments where stack memory is constrained, you might want to compile
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PCRE2 to use heap memory instead of stack for remembering back-up points when
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<b>pcre2_match()</b> is running. This makes it run more slowly, however. Details
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of how to do this are given in the
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<a href="pcre2build.html"><b>pcre2build</b></a>
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documentation. When built in this way, instead of using the stack, PCRE2
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2014-10-20 19:28:49 +02:00
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gets memory for remembering backup points from the heap. By default, the memory
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is obtained by calling the system <b>malloc()</b> function, but you can arrange
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to supply your own memory management function. For details, see the section
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entitled
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2014-10-20 18:48:14 +02:00
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<a href="pcre2api.html#matchcontext">"The match context"</a>
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in the
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<a href="pcre2api.html"><b>pcre2api</b></a>
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documentation. Since the block sizes are always the same, it may be possible to
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implement customized a memory handler that is more efficient than the standard
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function. The memory blocks obtained for this purpose are retained and re-used
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if possible while <b>pcre2_match()</b> is running. They are all freed just
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2014-10-20 18:48:14 +02:00
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before it exits.
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</P>
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<br><b>
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Limiting <b>pcre2_match()</b>'s stack usage
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</b><br>
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<P>
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You can set limits on the number of times the internal <b>match()</b> function
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is called, both in total and recursively. If a limit is exceeded,
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<b>pcre2_match()</b> returns an error code. Setting suitable limits should
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prevent it from running out of stack. The default values of the limits are very
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large, and unlikely ever to operate. They can be changed when PCRE2 is built,
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and they can also be set when <b>pcre2_match()</b> is called. For details of
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these interfaces, see the
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<a href="pcre2build.html"><b>pcre2build</b></a>
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documentation and the section entitled
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<a href="pcre2api.html#matchcontext">"The match context"</a>
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in the
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<a href="pcre2api.html"><b>pcre2api</b></a>
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documentation.
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</P>
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<P>
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As a very rough rule of thumb, you should reckon on about 500 bytes per
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recursion. Thus, if you want to limit your stack usage to 8Mb, you should set
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the limit at 16000 recursions. A 64Mb stack, on the other hand, can support
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around 128000 recursions.
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</P>
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<P>
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The <b>pcre2test</b> test program has a modifier called "find_limits" which, if
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applied to a subject line, causes it to find the smallest limits that allow a a
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pattern to match. This is done by calling <b>pcre2_match()</b> repeatedly with
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different limits.
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</P>
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<br><b>
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Changing stack size in Unix-like systems
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</b><br>
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<P>
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In Unix-like environments, there is not often a problem with the stack unless
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very long strings are involved, though the default limit on stack size varies
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from system to system. Values from 8Mb to 64Mb are common. You can find your
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default limit by running the command:
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<pre>
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ulimit -s
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</pre>
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Unfortunately, the effect of running out of stack is often SIGSEGV, though
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sometimes a more explicit error message is given. You can normally increase the
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limit on stack size by code such as this:
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<pre>
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struct rlimit rlim;
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getrlimit(RLIMIT_STACK, &rlim);
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rlim.rlim_cur = 100*1024*1024;
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setrlimit(RLIMIT_STACK, &rlim);
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</pre>
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This reads the current limits (soft and hard) using <b>getrlimit()</b>, then
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attempts to increase the soft limit to 100Mb using <b>setrlimit()</b>. You must
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do this before calling <b>pcre2_match()</b>.
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</P>
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<br><b>
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Changing stack size in Mac OS X
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</b><br>
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<P>
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Using <b>setrlimit()</b>, as described above, should also work on Mac OS X. It
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is also possible to set a stack size when linking a program. There is a
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discussion about stack sizes in Mac OS X at this web site:
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<a href="http://developer.apple.com/qa/qa2005/qa1419.html">http://developer.apple.com/qa/qa2005/qa1419.html.</a>
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</P>
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<br><b>
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AUTHOR
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</b><br>
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<P>
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Philip Hazel
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<br>
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University Computing Service
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<br>
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Cambridge CB2 3QH, England.
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<br>
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</P>
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<br><b>
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REVISION
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</b><br>
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<P>
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Last updated: 20 October 2014
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<br>
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Copyright © 1997-2014 University of Cambridge.
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<br>
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<p>
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Return to the <a href="index.html">PCRE2 index page</a>.
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</p>
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