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    1 <html>
    2 <head>
    3 <title>pcrejit specification</title>
    4 </head>
    5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
    6 <h1>pcrejit 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 <ul>
   16 <li><a name="TOC1" href="#SEC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a>
   17 <li><a name="TOC2" href="#SEC2">8-BIT, 16-BIT AND 32-BIT SUPPORT</a>
   18 <li><a name="TOC3" href="#SEC3">AVAILABILITY OF JIT SUPPORT</a>
   19 <li><a name="TOC4" href="#SEC4">SIMPLE USE OF JIT</a>
   20 <li><a name="TOC5" href="#SEC5">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a>
   21 <li><a name="TOC6" href="#SEC6">RETURN VALUES FROM JIT EXECUTION</a>
   22 <li><a name="TOC7" href="#SEC7">SAVING AND RESTORING COMPILED PATTERNS</a>
   23 <li><a name="TOC8" href="#SEC8">CONTROLLING THE JIT STACK</a>
   24 <li><a name="TOC9" href="#SEC9">JIT STACK FAQ</a>
   25 <li><a name="TOC10" href="#SEC10">EXAMPLE CODE</a>
   26 <li><a name="TOC11" href="#SEC11">JIT FAST PATH API</a>
   27 <li><a name="TOC12" href="#SEC12">SEE ALSO</a>
   28 <li><a name="TOC13" href="#SEC13">AUTHOR</a>
   29 <li><a name="TOC14" href="#SEC14">REVISION</a>
   30 </ul>
   31 <br><a name="SEC1" href="#TOC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a><br>
   32 <P>
   33 Just-in-time compiling is a heavyweight optimization that can greatly speed up
   34 pattern matching. However, it comes at the cost of extra processing before the
   35 match is performed. Therefore, it is of most benefit when the same pattern is
   36 going to be matched many times. This does not necessarily mean many calls of a
   37 matching function; if the pattern is not anchored, matching attempts may take
   38 place many times at various positions in the subject, even for a single call.
   39 Therefore, if the subject string is very long, it may still pay to use JIT for
   40 one-off matches.
   41 </P>
   42 <P>
   43 JIT support applies only to the traditional Perl-compatible matching function.
   44 It does not apply when the DFA matching function is being used. The code for
   45 this support was written by Zoltan Herczeg.
   46 </P>
   47 <br><a name="SEC2" href="#TOC1">8-BIT, 16-BIT AND 32-BIT SUPPORT</a><br>
   48 <P>
   49 JIT support is available for all of the 8-bit, 16-bit and 32-bit PCRE
   50 libraries. To keep this documentation simple, only the 8-bit interface is
   51 described in what follows. If you are using the 16-bit library, substitute the
   52 16-bit functions and 16-bit structures (for example, <i>pcre16_jit_stack</i>
   53 instead of <i>pcre_jit_stack</i>). If you are using the 32-bit library,
   54 substitute the 32-bit functions and 32-bit structures (for example,
   55 <i>pcre32_jit_stack</i> instead of <i>pcre_jit_stack</i>).
   56 </P>
   57 <br><a name="SEC3" href="#TOC1">AVAILABILITY OF JIT SUPPORT</a><br>
   58 <P>
   59 JIT support is an optional feature of PCRE. The "configure" option --enable-jit
   60 (or equivalent CMake option) must be set when PCRE is built if you want to use
   61 JIT. The support is limited to the following hardware platforms:
   62 <pre>
   63   ARM v5, v7, and Thumb2
   64   Intel x86 32-bit and 64-bit
   65   MIPS 32-bit
   66   Power PC 32-bit and 64-bit
   67   SPARC 32-bit (experimental)
   68 </pre>
   69 If --enable-jit is set on an unsupported platform, compilation fails.
   70 </P>
   71 <P>
   72 A program that is linked with PCRE 8.20 or later can tell if JIT support is
   73 available by calling <b>pcre_config()</b> with the PCRE_CONFIG_JIT option. The
   74 result is 1 when JIT is available, and 0 otherwise. However, a simple program
   75 does not need to check this in order to use JIT. The normal API is implemented
   76 in a way that falls back to the interpretive code if JIT is not available. For
   77 programs that need the best possible performance, there is also a "fast path"
   78 API that is JIT-specific.
   79 </P>
   80 <P>
   81 If your program may sometimes be linked with versions of PCRE that are older
   82 than 8.20, but you want to use JIT when it is available, you can test the
   83 values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT macro such as
   84 PCRE_CONFIG_JIT, for compile-time control of your code. Also beware that the
   85 <b>pcre_jit_exec()</b> function was not available at all before 8.32,
   86 and may not be available at all if PCRE isn't compiled with
   87 --enable-jit. See the "JIT FAST PATH API" section below for details.
   88 </P>
   89 <br><a name="SEC4" href="#TOC1">SIMPLE USE OF JIT</a><br>
   90 <P>
   91 You have to do two things to make use of the JIT support in the simplest way:
   92 <pre>
   93   (1) Call <b>pcre_study()</b> with the PCRE_STUDY_JIT_COMPILE option for
   94       each compiled pattern, and pass the resulting <b>pcre_extra</b> block to
   95       <b>pcre_exec()</b>.
   97   (2) Use <b>pcre_free_study()</b> to free the <b>pcre_extra</b> block when it is
   98       no longer needed, instead of just freeing it yourself. This ensures that
   99       any JIT data is also freed.
  100 </pre>
  101 For a program that may be linked with pre-8.20 versions of PCRE, you can insert
  102 <pre>
  103   #ifndef PCRE_STUDY_JIT_COMPILE
  104   #define PCRE_STUDY_JIT_COMPILE 0
  105   #endif
  106 </pre>
  107 so that no option is passed to <b>pcre_study()</b>, and then use something like
  108 this to free the study data:
  109 <pre>
  110   #ifdef PCRE_CONFIG_JIT
  111       pcre_free_study(study_ptr);
  112   #else
  113       pcre_free(study_ptr);
  114   #endif
  115 </pre>
  116 PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for complete
  117 matches. If you want to run partial matches using the PCRE_PARTIAL_HARD or
  118 PCRE_PARTIAL_SOFT options of <b>pcre_exec()</b>, you should set one or both of
  119 the following options in addition to, or instead of, PCRE_STUDY_JIT_COMPILE
  120 when you call <b>pcre_study()</b>:
  121 <pre>
  124 </pre>
  125 If using <b>pcre_jit_exec()</b> and supporting a pre-8.32 version of
  126 PCRE, you can insert:
  127 <pre>
  128    #if PCRE_MAJOR &#62;= 8 && PCRE_MINOR &#62;= 32
  129    pcre_jit_exec(...);
  130    #else
  131    pcre_exec(...)
  132    #endif
  133 </pre>
  134 but as described in the "JIT FAST PATH API" section below this assumes
  135 version 8.32 and later are compiled with --enable-jit, which may
  136 break.
  137 <br>
  138 <br>
  139 The JIT compiler generates different optimized code for each of the three
  140 modes (normal, soft partial, hard partial). When <b>pcre_exec()</b> is called,
  141 the appropriate code is run if it is available. Otherwise, the pattern is
  142 matched using interpretive code.
  143 </P>
  144 <P>
  145 In some circumstances you may need to call additional functions. These are
  146 described in the section entitled
  147 <a href="#stackcontrol">"Controlling the JIT stack"</a>
  148 below.
  149 </P>
  150 <P>
  151 If JIT support is not available, PCRE_STUDY_JIT_COMPILE etc. are ignored, and
  152 no JIT data is created. Otherwise, the compiled pattern is passed to the JIT
  153 compiler, which turns it into machine code that executes much faster than the
  154 normal interpretive code. When <b>pcre_exec()</b> is passed a <b>pcre_extra</b>
  155 block containing a pointer to JIT code of the appropriate mode (normal or
  156 hard/soft partial), it obeys that code instead of running the interpreter. The
  157 result is identical, but the compiled JIT code runs much faster.
  158 </P>
  159 <P>
  160 There are some <b>pcre_exec()</b> options that are not supported for JIT
  161 execution. There are also some pattern items that JIT cannot handle. Details
  162 are given below. In both cases, execution automatically falls back to the
  163 interpretive code. If you want to know whether JIT was actually used for a
  164 particular match, you should arrange for a JIT callback function to be set up
  165 as described in the section entitled
  166 <a href="#stackcontrol">"Controlling the JIT stack"</a>
  167 below, even if you do not need to supply a non-default JIT stack. Such a
  168 callback function is called whenever JIT code is about to be obeyed. If the
  169 execution options are not right for JIT execution, the callback function is not
  170 obeyed.
  171 </P>
  172 <P>
  173 If the JIT compiler finds an unsupported item, no JIT data is generated. You
  174 can find out if JIT execution is available after studying a pattern by calling
  175 <b>pcre_fullinfo()</b> with the PCRE_INFO_JIT option. A result of 1 means that
  176 JIT compilation was successful. A result of 0 means that JIT support is not
  177 available, or the pattern was not studied with PCRE_STUDY_JIT_COMPILE etc., or
  178 the JIT compiler was not able to handle the pattern.
  179 </P>
  180 <P>
  181 Once a pattern has been studied, with or without JIT, it can be used as many
  182 times as you like for matching different subject strings.
  183 </P>
  184 <br><a name="SEC5" href="#TOC1">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a><br>
  185 <P>
  186 The only <b>pcre_exec()</b> options that are supported for JIT execution are
  190 </P>
  191 <P>
  192 The only unsupported pattern items are \C (match a single data unit) when
  193 running in a UTF mode, and a callout immediately before an assertion condition
  194 in a conditional group.
  195 </P>
  196 <br><a name="SEC6" href="#TOC1">RETURN VALUES FROM JIT EXECUTION</a><br>
  197 <P>
  198 When a pattern is matched using JIT execution, the return values are the same
  199 as those given by the interpretive <b>pcre_exec()</b> code, with the addition of
  200 one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means that the memory used
  201 for the JIT stack was insufficient. See
  202 <a href="#stackcontrol">"Controlling the JIT stack"</a>
  203 below for a discussion of JIT stack usage. For compatibility with the
  204 interpretive <b>pcre_exec()</b> code, no more than two-thirds of the
  205 <i>ovector</i> argument is used for passing back captured substrings.
  206 </P>
  207 <P>
  208 The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if searching a
  209 very large pattern tree goes on for too long, as it is in the same circumstance
  210 when JIT is not used, but the details of exactly what is counted are not the
  211 same. The PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT
  212 execution.
  213 </P>
  214 <br><a name="SEC7" href="#TOC1">SAVING AND RESTORING COMPILED PATTERNS</a><br>
  215 <P>
  216 The code that is generated by the JIT compiler is architecture-specific, and is
  217 also position dependent. For those reasons it cannot be saved (in a file or
  218 database) and restored later like the bytecode and other data of a compiled
  219 pattern. Saving and restoring compiled patterns is not something many people
  220 do. More detail about this facility is given in the
  221 <a href="pcreprecompile.html"><b>pcreprecompile</b></a>
  222 documentation. It should be possible to run <b>pcre_study()</b> on a saved and
  223 restored pattern, and thereby recreate the JIT data, but because JIT
  224 compilation uses significant resources, it is probably not worth doing this;
  225 you might as well recompile the original pattern.
  226 <a name="stackcontrol"></a></P>
  227 <br><a name="SEC8" href="#TOC1">CONTROLLING THE JIT STACK</a><br>
  228 <P>
  229 When the compiled JIT code runs, it needs a block of memory to use as a stack.
  230 By default, it uses 32K on the machine stack. However, some large or
  231 complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT
  232 is given when there is not enough stack. Three functions are provided for
  233 managing blocks of memory for use as JIT stacks. There is further discussion
  234 about the use of JIT stacks in the section entitled
  235 <a href="#stackcontrol">"JIT stack FAQ"</a>
  236 below.
  237 </P>
  238 <P>
  239 The <b>pcre_jit_stack_alloc()</b> function creates a JIT stack. Its arguments
  240 are a starting size and a maximum size, and it returns a pointer to an opaque
  241 structure of type <b>pcre_jit_stack</b>, or NULL if there is an error. The
  242 <b>pcre_jit_stack_free()</b> function can be used to free a stack that is no
  243 longer needed. (For the technically minded: the address space is allocated by
  244 mmap or VirtualAlloc.)
  245 </P>
  246 <P>
  247 JIT uses far less memory for recursion than the interpretive code,
  248 and a maximum stack size of 512K to 1M should be more than enough for any
  249 pattern.
  250 </P>
  251 <P>
  252 The <b>pcre_assign_jit_stack()</b> function specifies which stack JIT code
  253 should use. Its arguments are as follows:
  254 <pre>
  255   pcre_extra         *extra
  256   pcre_jit_callback  callback
  257   void               *data
  258 </pre>
  259 The <i>extra</i> argument must be the result of studying a pattern with
  260 PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of the other
  261 two options:
  262 <pre>
  263   (1) If <i>callback</i> is NULL and <i>data</i> is NULL, an internal 32K block
  264       on the machine stack is used.
  266   (2) If <i>callback</i> is NULL and <i>data</i> is not NULL, <i>data</i> must be
  267       a valid JIT stack, the result of calling <b>pcre_jit_stack_alloc()</b>.
  269   (3) If <i>callback</i> is not NULL, it must point to a function that is
  270       called with <i>data</i> as an argument at the start of matching, in
  271       order to set up a JIT stack. If the return from the callback
  272       function is NULL, the internal 32K stack is used; otherwise the
  273       return value must be a valid JIT stack, the result of calling
  274       <b>pcre_jit_stack_alloc()</b>.
  275 </pre>
  276 A callback function is obeyed whenever JIT code is about to be run; it is not
  277 obeyed when <b>pcre_exec()</b> is called with options that are incompatible for
  278 JIT execution. A callback function can therefore be used to determine whether a
  279 match operation was executed by JIT or by the interpreter.
  280 </P>
  281 <P>
  282 You may safely use the same JIT stack for more than one pattern (either by
  283 assigning directly or by callback), as long as the patterns are all matched
  284 sequentially in the same thread. In a multithread application, if you do not
  285 specify a JIT stack, or if you assign or pass back NULL from a callback, that
  286 is thread-safe, because each thread has its own machine stack. However, if you
  287 assign or pass back a non-NULL JIT stack, this must be a different stack for
  288 each thread so that the application is thread-safe.
  289 </P>
  290 <P>
  291 Strictly speaking, even more is allowed. You can assign the same non-NULL stack
  292 to any number of patterns as long as they are not used for matching by multiple
  293 threads at the same time. For example, you can assign the same stack to all
  294 compiled patterns, and use a global mutex in the callback to wait until the
  295 stack is available for use. However, this is an inefficient solution, and not
  296 recommended.
  297 </P>
  298 <P>
  299 This is a suggestion for how a multithreaded program that needs to set up
  300 non-default JIT stacks might operate:
  301 <pre>
  302   During thread initalization
  303     thread_local_var = pcre_jit_stack_alloc(...)
  305   During thread exit
  306     pcre_jit_stack_free(thread_local_var)
  308   Use a one-line callback function
  309     return thread_local_var
  310 </pre>
  311 All the functions described in this section do nothing if JIT is not available,
  312 and <b>pcre_assign_jit_stack()</b> does nothing unless the <b>extra</b> argument
  313 is non-NULL and points to a <b>pcre_extra</b> block that is the result of a
  314 successful study with PCRE_STUDY_JIT_COMPILE etc.
  315 <a name="stackfaq"></a></P>
  316 <br><a name="SEC9" href="#TOC1">JIT STACK FAQ</a><br>
  317 <P>
  318 (1) Why do we need JIT stacks?
  319 <br>
  320 <br>
  321 PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack where
  322 the local data of the current node is pushed before checking its child nodes.
  323 Allocating real machine stack on some platforms is difficult. For example, the
  324 stack chain needs to be updated every time if we extend the stack on PowerPC.
  325 Although it is possible, its updating time overhead decreases performance. So
  326 we do the recursion in memory.
  327 </P>
  328 <P>
  329 (2) Why don't we simply allocate blocks of memory with <b>malloc()</b>?
  330 <br>
  331 <br>
  332 Modern operating systems have a nice feature: they can reserve an address space
  333 instead of allocating memory. We can safely allocate memory pages inside this
  334 address space, so the stack could grow without moving memory data (this is
  335 important because of pointers). Thus we can allocate 1M address space, and use
  336 only a single memory page (usually 4K) if that is enough. However, we can still
  337 grow up to 1M anytime if needed.
  338 </P>
  339 <P>
  340 (3) Who "owns" a JIT stack?
  341 <br>
  342 <br>
  343 The owner of the stack is the user program, not the JIT studied pattern or
  344 anything else. The user program must ensure that if a stack is used by
  345 <b>pcre_exec()</b>, (that is, it is assigned to the pattern currently running),
  346 that stack must not be used by any other threads (to avoid overwriting the same
  347 memory area). The best practice for multithreaded programs is to allocate a
  348 stack for each thread, and return this stack through the JIT callback function.
  349 </P>
  350 <P>
  351 (4) When should a JIT stack be freed?
  352 <br>
  353 <br>
  354 You can free a JIT stack at any time, as long as it will not be used by
  355 <b>pcre_exec()</b> again. When you assign the stack to a pattern, only a pointer
  356 is set. There is no reference counting or any other magic. You can free the
  357 patterns and stacks in any order, anytime. Just <i>do not</i> call
  358 <b>pcre_exec()</b> with a pattern pointing to an already freed stack, as that
  359 will cause SEGFAULT. (Also, do not free a stack currently used by
  360 <b>pcre_exec()</b> in another thread). You can also replace the stack for a
  361 pattern at any time. You can even free the previous stack before assigning a
  362 replacement.
  363 </P>
  364 <P>
  365 (5) Should I allocate/free a stack every time before/after calling
  366 <b>pcre_exec()</b>?
  367 <br>
  368 <br>
  369 No, because this is too costly in terms of resources. However, you could
  370 implement some clever idea which release the stack if it is not used in let's
  371 say two minutes. The JIT callback can help to achieve this without keeping a
  372 list of the currently JIT studied patterns.
  373 </P>
  374 <P>
  375 (6) OK, the stack is for long term memory allocation. But what happens if a
  376 pattern causes stack overflow with a stack of 1M? Is that 1M kept until the
  377 stack is freed?
  378 <br>
  379 <br>
  380 Especially on embedded sytems, it might be a good idea to release memory
  381 sometimes without freeing the stack. There is no API for this at the moment.
  382 Probably a function call which returns with the currently allocated memory for
  383 any stack and another which allows releasing memory (shrinking the stack) would
  384 be a good idea if someone needs this.
  385 </P>
  386 <P>
  387 (7) This is too much of a headache. Isn't there any better solution for JIT
  388 stack handling?
  389 <br>
  390 <br>
  391 No, thanks to Windows. If POSIX threads were used everywhere, we could throw
  392 out this complicated API.
  393 </P>
  394 <br><a name="SEC10" href="#TOC1">EXAMPLE CODE</a><br>
  395 <P>
  396 This is a single-threaded example that specifies a JIT stack without using a
  397 callback.
  398 <pre>
  399   int rc;
  400   int ovector[30];
  401   pcre *re;
  402   pcre_extra *extra;
  403   pcre_jit_stack *jit_stack;
  405   re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
  406   /* Check for errors */
  407   extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
  408   jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024);
  409   /* Check for error (NULL) */
  410   pcre_assign_jit_stack(extra, NULL, jit_stack);
  411   rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
  412   /* Check results */
  413   pcre_free(re);
  414   pcre_free_study(extra);
  415   pcre_jit_stack_free(jit_stack);
  417 </PRE>
  418 </P>
  419 <br><a name="SEC11" href="#TOC1">JIT FAST PATH API</a><br>
  420 <P>
  421 Because the API described above falls back to interpreted execution when JIT is
  422 not available, it is convenient for programs that are written for general use
  423 in many environments. However, calling JIT via <b>pcre_exec()</b> does have a
  424 performance impact. Programs that are written for use where JIT is known to be
  425 available, and which need the best possible performance, can instead use a
  426 "fast path" API to call JIT execution directly instead of calling
  427 <b>pcre_exec()</b> (obviously only for patterns that have been successfully
  428 studied by JIT).
  429 </P>
  430 <P>
  431 The fast path function is called <b>pcre_jit_exec()</b>, and it takes exactly
  432 the same arguments as <b>pcre_exec()</b>, plus one additional argument that
  433 must point to a JIT stack. The JIT stack arrangements described above do not
  434 apply. The return values are the same as for <b>pcre_exec()</b>.
  435 </P>
  436 <P>
  437 When you call <b>pcre_exec()</b>, as well as testing for invalid options, a
  438 number of other sanity checks are performed on the arguments. For example, if
  439 the subject pointer is NULL, or its length is negative, an immediate error is
  440 given. Also, unless PCRE_NO_UTF[8|16|32] is set, a UTF subject string is tested
  441 for validity. In the interests of speed, these checks do not happen on the JIT
  442 fast path, and if invalid data is passed, the result is undefined.
  443 </P>
  444 <P>
  445 Bypassing the sanity checks and the <b>pcre_exec()</b> wrapping can give
  446 speedups of more than 10%.
  447 </P>
  448 <P>
  449 Note that the <b>pcre_jit_exec()</b> function is not available in versions of
  450 PCRE before 8.32 (released in November 2012). If you need to support versions
  451 that old you must either use the slower <b>pcre_exec()</b>, or switch between
  452 the two codepaths by checking the values of PCRE_MAJOR and PCRE_MINOR.
  453 </P>
  454 <P>
  455 Due to an unfortunate implementation oversight, even in versions 8.32
  456 and later there will be no <b>pcre_jit_exec()</b> stub function defined
  457 when PCRE is compiled with --disable-jit, which is the default, and
  458 there's no way to detect whether PCRE was compiled with --enable-jit
  459 via a macro.
  460 </P>
  461 <P>
  462 If you need to support versions older than 8.32, or versions that may
  463 not build with --enable-jit, you must either use the slower
  464 <b>pcre_exec()</b>, or switch between the two codepaths by checking the
  465 values of PCRE_MAJOR and PCRE_MINOR.
  466 </P>
  467 <P>
  468 Switching between the two by checking the version assumes that all the
  469 versions being targeted are built with --enable-jit. To also support
  470 builds that may use --disable-jit either <b>pcre_exec()</b> must be
  471 used, or a compile-time check for JIT via <b>pcre_config()</b> (which
  472 assumes the runtime environment will be the same), or as the Git
  473 project decided to do, simply assume that <b>pcre_jit_exec()</b> is
  474 present in 8.32 or later unless a compile-time flag is provided, see
  475 the "grep: un-break building with PCRE &#62;= 8.32 without --enable-jit"
  476 commit in git.git for an example of that.
  477 </P>
  478 <br><a name="SEC12" href="#TOC1">SEE ALSO</a><br>
  479 <P>
  480 <b>pcreapi</b>(3)
  481 </P>
  482 <br><a name="SEC13" href="#TOC1">AUTHOR</a><br>
  483 <P>
  484 Philip Hazel (FAQ by Zoltan Herczeg)
  485 <br>
  486 University Computing Service
  487 <br>
  488 Cambridge CB2 3QH, England.
  489 <br>
  490 </P>
  491 <br><a name="SEC14" href="#TOC1">REVISION</a><br>
  492 <P>
  493 Last updated: 05 July 2017
  494 <br>
  495 Copyright &copy; 1997-2017 University of Cambridge.
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