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    1 Technical Notes about PCRE
    2 --------------------------
    3 
    4 These are very rough technical notes that record potentially useful information 
    5 about PCRE internals. For information about testing PCRE, see the pcretest 
    6 documentation and the comment at the head of the RunTest file.
    7 
    8 
    9 Historical note 1
   10 -----------------
   11 
   12 Many years ago I implemented some regular expression functions to an algorithm
   13 suggested by Martin Richards. These were not Unix-like in form, and were quite
   14 restricted in what they could do by comparison with Perl. The interesting part
   15 about the algorithm was that the amount of space required to hold the compiled
   16 form of an expression was known in advance. The code to apply an expression did
   17 not operate by backtracking, as the original Henry Spencer code and current
   18 Perl code does, but instead checked all possibilities simultaneously by keeping
   19 a list of current states and checking all of them as it advanced through the
   20 subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA
   21 algorithm", though it was not a traditional Finite State Machine (FSM). When
   22 the pattern was all used up, all remaining states were possible matches, and
   23 the one matching the longest subset of the subject string was chosen. This did
   24 not necessarily maximize the individual wild portions of the pattern, as is
   25 expected in Unix and Perl-style regular expressions.
   26 
   27 
   28 Historical note 2
   29 -----------------
   30 
   31 By contrast, the code originally written by Henry Spencer (which was
   32 subsequently heavily modified for Perl) compiles the expression twice: once in
   33 a dummy mode in order to find out how much store will be needed, and then for
   34 real. (The Perl version probably doesn't do this any more; I'm talking about
   35 the original library.) The execution function operates by backtracking and
   36 maximizing (or, optionally, minimizing in Perl) the amount of the subject that
   37 matches individual wild portions of the pattern. This is an "NFA algorithm" in
   38 Friedl's terminology.
   39 
   40 
   41 OK, here's the real stuff
   42 -------------------------
   43 
   44 For the set of functions that form the "basic" PCRE library (which are
   45 unrelated to those mentioned above), I tried at first to invent an algorithm
   46 that used an amount of store bounded by a multiple of the number of characters
   47 in the pattern, to save on compiling time. However, because of the greater
   48 complexity in Perl regular expressions, I couldn't do this. In any case, a
   49 first pass through the pattern is helpful for other reasons. 
   50 
   51 
   52 Support for 16-bit and 32-bit data strings
   53 -------------------------------------------
   54 
   55 From release 8.30, PCRE supports 16-bit as well as 8-bit data strings; and from
   56 release 8.32, PCRE supports 32-bit data strings. The library can be compiled
   57 in any combination of 8-bit, 16-bit or 32-bit modes, creating up to three
   58 different libraries. In the description that follows, the word "short" is used
   59 for a 16-bit data quantity, and the word "unit" is used for a quantity that is
   60 a byte in 8-bit mode, a short in 16-bit mode and a 32-bit word in 32-bit mode.
   61 However, so as not to over-complicate the text, the names of PCRE functions are
   62 given in 8-bit form only.
   63 
   64 
   65 Computing the memory requirement: how it was
   66 --------------------------------------------
   67 
   68 Up to and including release 6.7, PCRE worked by running a very degenerate first
   69 pass to calculate a maximum store size, and then a second pass to do the real
   70 compile - which might use a bit less than the predicted amount of memory. The
   71 idea was that this would turn out faster than the Henry Spencer code because
   72 the first pass is degenerate and the second pass can just store stuff straight
   73 into the vector, which it knows is big enough.
   74 
   75 
   76 Computing the memory requirement: how it is
   77 -------------------------------------------
   78 
   79 By the time I was working on a potential 6.8 release, the degenerate first pass
   80 had become very complicated and hard to maintain. Indeed one of the early
   81 things I did for 6.8 was to fix Yet Another Bug in the memory computation. Then
   82 I had a flash of inspiration as to how I could run the real compile function in
   83 a "fake" mode that enables it to compute how much memory it would need, while
   84 actually only ever using a few hundred bytes of working memory, and without too
   85 many tests of the mode that might slow it down. So I refactored the compiling
   86 functions to work this way. This got rid of about 600 lines of source. It
   87 should make future maintenance and development easier. As this was such a major 
   88 change, I never released 6.8, instead upping the number to 7.0 (other quite 
   89 major changes were also present in the 7.0 release).
   90 
   91 A side effect of this work was that the previous limit of 200 on the nesting
   92 depth of parentheses was removed. However, there is a downside: pcre_compile()
   93 runs more slowly than before (30% or more, depending on the pattern) because it
   94 is doing a full analysis of the pattern. My hope was that this would not be a
   95 big issue, and in the event, nobody has commented on it.
   96 
   97 At release 8.34, a limit on the nesting depth of parentheses was re-introduced
   98 (default 250, settable at build time) so as to put a limit on the amount of 
   99 system stack used by pcre_compile(). This is a safety feature for environments 
  100 with small stacks where the patterns are provided by users.
  101 
  102 
  103 Traditional matching function
  104 -----------------------------
  105 
  106 The "traditional", and original, matching function is called pcre_exec(), and 
  107 it implements an NFA algorithm, similar to the original Henry Spencer algorithm 
  108 and the way that Perl works. This is not surprising, since it is intended to be
  109 as compatible with Perl as possible. This is the function most users of PCRE
  110 will use most of the time. From release 8.20, if PCRE is compiled with 
  111 just-in-time (JIT) support, and studying a compiled pattern with JIT is 
  112 successful, the JIT code is run instead of the normal pcre_exec() code, but the 
  113 result is the same.
  114 
  115 
  116 Supplementary matching function
  117 -------------------------------
  118 
  119 From PCRE 6.0, there is also a supplementary matching function called 
  120 pcre_dfa_exec(). This implements a DFA matching algorithm that searches 
  121 simultaneously for all possible matches that start at one point in the subject 
  122 string. (Going back to my roots: see Historical Note 1 above.) This function 
  123 intreprets the same compiled pattern data as pcre_exec(); however, not all the 
  124 facilities are available, and those that are do not always work in quite the 
  125 same way. See the user documentation for details.
  126 
  127 The algorithm that is used for pcre_dfa_exec() is not a traditional FSM, 
  128 because it may have a number of states active at one time. More work would be
  129 needed at compile time to produce a traditional FSM where only one state is
  130 ever active at once. I believe some other regex matchers work this way. JIT
  131 support is not available for this kind of matching.
  132 
  133 
  134 Changeable options
  135 ------------------
  136 
  137 The /i, /m, or /s options (PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and some 
  138 others) may change in the middle of patterns. From PCRE 8.13, their processing
  139 is handled entirely at compile time by generating different opcodes for the
  140 different settings. The runtime functions do not need to keep track of an
  141 options state any more.
  142 
  143 
  144 Format of compiled patterns
  145 ---------------------------
  146 
  147 The compiled form of a pattern is a vector of unsigned units (bytes in 8-bit
  148 mode, shorts in 16-bit mode, 32-bit words in 32-bit mode), containing items of
  149 variable length. The first unit in an item contains an opcode, and the length
  150 of the item is either implicit in the opcode or contained in the data that
  151 follows it.
  152 
  153 In many cases listed below, LINK_SIZE data values are specified for offsets
  154 within the compiled pattern. LINK_SIZE always specifies a number of bytes. The
  155 default value for LINK_SIZE is 2, but PCRE can be compiled to use 3-byte or
  156 4-byte values for these offsets, although this impairs the performance. (3-byte
  157 LINK_SIZE values are available only in 8-bit mode.) Specifing a LINK_SIZE
  158 larger than 2 is necessary only when patterns whose compiled length is greater
  159 than 64K are going to be processed. In this description, we assume the "normal"
  160 compilation options. Data values that are counts (e.g. quantifiers) are two
  161 bytes long in 8-bit mode (most significant byte first), or one unit in 16-bit
  162 and 32-bit modes.
  163 
  164 
  165 Opcodes with no following data
  166 ------------------------------
  167 
  168 These items are all just one unit long
  169 
  170   OP_END                 end of pattern
  171   OP_ANY                 match any one character other than newline
  172   OP_ALLANY              match any one character, including newline
  173   OP_ANYBYTE             match any single unit, even in UTF-8/16 mode
  174   OP_SOD                 match start of data: \A
  175   OP_SOM,                start of match (subject + offset): \G
  176   OP_SET_SOM,            set start of match (\K) 
  177   OP_CIRC                ^ (start of data)
  178   OP_CIRCM               ^ multiline mode (start of data or after newline)
  179   OP_NOT_WORD_BOUNDARY   \W
  180   OP_WORD_BOUNDARY       \w
  181   OP_NOT_DIGIT           \D
  182   OP_DIGIT               \d
  183   OP_NOT_HSPACE          \H
  184   OP_HSPACE              \h  
  185   OP_NOT_WHITESPACE      \S
  186   OP_WHITESPACE          \s
  187   OP_NOT_VSPACE          \V
  188   OP_VSPACE              \v  
  189   OP_NOT_WORDCHAR        \W
  190   OP_WORDCHAR            \w
  191   OP_EODN                match end of data or newline at end: \Z
  192   OP_EOD                 match end of data: \z
  193   OP_DOLL                $ (end of data, or before final newline)
  194   OP_DOLLM               $ multiline mode (end of data or before newline)
  195   OP_EXTUNI              match an extended Unicode grapheme cluster 
  196   OP_ANYNL               match any Unicode newline sequence 
  197   
  198   OP_ASSERT_ACCEPT       )
  199   OP_ACCEPT              ) These are Perl 5.10's "backtracking control   
  200   OP_COMMIT              ) verbs". If OP_ACCEPT is inside capturing
  201   OP_FAIL                ) parentheses, it may be preceded by one or more
  202   OP_PRUNE               ) OP_CLOSE, each followed by a count that
  203   OP_SKIP                ) indicates which parentheses must be closed.
  204   OP_THEN                )
  205   
  206 OP_ASSERT_ACCEPT is used when (*ACCEPT) is encountered within an assertion. 
  207 This ends the assertion, not the entire pattern match.  
  208   
  209 
  210 Backtracking control verbs with optional data
  211 ---------------------------------------------
  212 
  213 (*THEN) without an argument generates the opcode OP_THEN and no following data.
  214 OP_MARK is followed by the mark name, preceded by a one-unit length, and
  215 followed by a binary zero. For (*PRUNE), (*SKIP), and (*THEN) with arguments,
  216 the opcodes OP_PRUNE_ARG, OP_SKIP_ARG, and OP_THEN_ARG are used, with the name
  217 following in the same format as OP_MARK.
  218   
  219 
  220 Matching literal characters
  221 ---------------------------
  222 
  223 The OP_CHAR opcode is followed by a single character that is to be matched 
  224 casefully. For caseless matching, OP_CHARI is used. In UTF-8 or UTF-16 modes,
  225 the character may be more than one unit long. In UTF-32 mode, characters
  226 are always exactly one unit long.
  227 
  228 If there is only one character in a character class, OP_CHAR or OP_CHARI is
  229 used for a positive class, and OP_NOT or OP_NOTI for a negative one (that is,
  230 for something like [^a]).
  231 
  232 
  233 Repeating single characters
  234 ---------------------------
  235 
  236 The common repeats (*, +, ?), when applied to a single character, use the
  237 following opcodes, which come in caseful and caseless versions:
  238 
  239   Caseful         Caseless
  240   OP_STAR         OP_STARI      
  241   OP_MINSTAR      OP_MINSTARI   
  242   OP_POSSTAR      OP_POSSTARI   
  243   OP_PLUS         OP_PLUSI      
  244   OP_MINPLUS      OP_MINPLUSI   
  245   OP_POSPLUS      OP_POSPLUSI   
  246   OP_QUERY        OP_QUERYI     
  247   OP_MINQUERY     OP_MINQUERYI  
  248   OP_POSQUERY     OP_POSQUERYI  
  249 
  250 Each opcode is followed by the character that is to be repeated. In ASCII mode,
  251 these are two-unit items; in UTF-8 or UTF-16 modes, the length is variable; in
  252 UTF-32 mode these are one-unit items. Those with "MIN" in their names are the
  253 minimizing versions. Those with "POS" in their names are possessive versions.
  254 Other repeats make use of these opcodes:
  255 
  256   Caseful         Caseless
  257   OP_UPTO         OP_UPTOI    
  258   OP_MINUPTO      OP_MINUPTOI 
  259   OP_POSUPTO      OP_POSUPTOI 
  260   OP_EXACT        OP_EXACTI   
  261 
  262 Each of these is followed by a count and then the repeated character. OP_UPTO
  263 matches from 0 to the given number. A repeat with a non-zero minimum and a
  264 fixed maximum is coded as an OP_EXACT followed by an OP_UPTO (or OP_MINUPTO or
  265 OPT_POSUPTO).
  266 
  267 Another set of matching repeating opcodes (called OP_NOTSTAR, OP_NOTSTARI,
  268 etc.) are used for repeated, negated, single-character classes such as [^a]*.
  269 The normal single-character opcodes (OP_STAR, etc.) are used for repeated
  270 positive single-character classes.
  271 
  272 
  273 Repeating character types
  274 -------------------------
  275 
  276 Repeats of things like \d are done exactly as for single characters, except
  277 that instead of a character, the opcode for the type is stored in the data
  278 unit. The opcodes are:
  279 
  280   OP_TYPESTAR
  281   OP_TYPEMINSTAR
  282   OP_TYPEPOSSTAR 
  283   OP_TYPEPLUS
  284   OP_TYPEMINPLUS
  285   OP_TYPEPOSPLUS 
  286   OP_TYPEQUERY
  287   OP_TYPEMINQUERY
  288   OP_TYPEPOSQUERY 
  289   OP_TYPEUPTO
  290   OP_TYPEMINUPTO
  291   OP_TYPEPOSUPTO 
  292   OP_TYPEEXACT
  293 
  294 
  295 Match by Unicode property
  296 -------------------------
  297 
  298 OP_PROP and OP_NOTPROP are used for positive and negative matches of a 
  299 character by testing its Unicode property (the \p and \P escape sequences).
  300 Each is followed by two units that encode the desired property as a type and a
  301 value. The types are a set of #defines of the form PT_xxx, and the values are
  302 enumerations of the form ucp_xx, defined in the ucp.h source file. The value is
  303 relevant only for PT_GC (General Category), PT_PC (Particular Category), and
  304 PT_SC (Script).
  305 
  306 Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by
  307 three units: OP_PROP or OP_NOTPROP, and then the desired property type and
  308 value.
  309 
  310 
  311 Character classes
  312 -----------------
  313 
  314 If there is only one character in a class, OP_CHAR or OP_CHARI is used for a
  315 positive class, and OP_NOT or OP_NOTI for a negative one (that is, for
  316 something like [^a]). 
  317 
  318 A set of repeating opcodes (called OP_NOTSTAR etc.) are used for repeated,
  319 negated, single-character classes. The normal single-character opcodes
  320 (OP_STAR, etc.) are used for repeated positive single-character classes.
  321 
  322 When there is more than one character in a class, and all the code points are
  323 less than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a
  324 negative one. In either case, the opcode is followed by a 32-byte (16-short, 
  325 8-word) bit map containing a 1 bit for every character that is acceptable. The
  326 bits are counted from the least significant end of each unit. In caseless mode,
  327 bits for both cases are set.
  328 
  329 The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8/16/32
  330 mode, subject characters with values greater than 255 can be handled correctly.
  331 For OP_CLASS they do not match, whereas for OP_NCLASS they do.
  332 
  333 For classes containing characters with values greater than 255 or that contain 
  334 \p or \P, OP_XCLASS is used. It optionally uses a bit map if any code points
  335 are less than 256, followed by a list of pairs (for a range) and single
  336 characters. In caseless mode, both cases are explicitly listed.
  337 
  338 OP_XCLASS is followed by a unit containing flag bits: XCL_NOT indicates that 
  339 this is a negative class, and XCL_MAP indicates that a bit map is present.
  340 There follows the bit map, if XCL_MAP is set, and then a sequence of items
  341 coded as follows:
  342 
  343   XCL_END      marks the end of the list
  344   XCL_SINGLE   one character follows
  345   XCL_RANGE    two characters follow
  346   XCL_PROP     a Unicode property (type, value) follows   
  347   XCL_NOTPROP  a Unicode property (type, value) follows   
  348 
  349 If a range starts with a code point less than 256 and ends with one greater 
  350 than 256, an XCL_RANGE item is used, without setting any bits in the bit map. 
  351 This means that if no other items in the class set bits in the map, a map is 
  352 not needed.
  353 
  354 
  355 Back references
  356 ---------------
  357 
  358 OP_REF (caseful) or OP_REFI (caseless) is followed by a count containing the
  359 reference number if the reference is to a unique capturing group (either by
  360 number or by name). When named groups are used, there may be more than one
  361 group with the same name. In this case, a reference by name generates OP_DNREF
  362 or OP_DNREFI. These are followed by two counts: the index (not the byte offset) 
  363 in the group name table of the first entry for the requred name, followed by
  364 the number of groups with the same name.
  365 
  366 
  367 Repeating character classes and back references
  368 -----------------------------------------------
  369 
  370 Single-character classes are handled specially (see above). This section
  371 applies to other classes and also to back references. In both cases, the repeat
  372 information follows the base item. The matching code looks at the following
  373 opcode to see if it is one of
  374 
  375   OP_CRSTAR
  376   OP_CRMINSTAR
  377   OP_CRPOSSTAR 
  378   OP_CRPLUS
  379   OP_CRMINPLUS
  380   OP_CRPOSPLUS 
  381   OP_CRQUERY
  382   OP_CRMINQUERY
  383   OP_CRPOSQUERY 
  384   OP_CRRANGE
  385   OP_CRMINRANGE
  386   OP_CRPOSRANGE 
  387 
  388 All but the last three are single-unit items, with no data. The others are
  389 followed by the minimum and maximum repeat counts.
  390 
  391 
  392 Brackets and alternation
  393 ------------------------
  394 
  395 A pair of non-capturing round brackets is wrapped round each expression at
  396 compile time, so alternation always happens in the context of brackets.
  397 
  398 [Note for North Americans: "bracket" to some English speakers, including
  399 myself, can be round, square, curly, or pointy. Hence this usage rather than 
  400 "parentheses".]
  401 
  402 Non-capturing brackets use the opcode OP_BRA. Originally PCRE was limited to 99
  403 capturing brackets and it used a different opcode for each one. From release
  404 3.5, the limit was removed by putting the bracket number into the data for
  405 higher-numbered brackets. From release 7.0 all capturing brackets are handled
  406 this way, using the single opcode OP_CBRA.
  407 
  408 A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
  409 next alternative OP_ALT or, if there aren't any branches, to the matching
  410 OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
  411 the next one, or to the OP_KET opcode. For capturing brackets, the bracket 
  412 number is a count that immediately follows the offset.
  413 
  414 OP_KET is used for subpatterns that do not repeat indefinitely, and OP_KETRMIN
  415 and OP_KETRMAX are used for indefinite repetitions, minimally or maximally
  416 respectively (see below for possessive repetitions). All three are followed by
  417 LINK_SIZE bytes giving (as a positive number) the offset back to the matching
  418 bracket opcode.
  419 
  420 If a subpattern is quantified such that it is permitted to match zero times, it
  421 is preceded by one of OP_BRAZERO, OP_BRAMINZERO, or OP_SKIPZERO. These are
  422 single-unit opcodes that tell the matcher that skipping the following
  423 subpattern entirely is a valid branch. In the case of the first two, not 
  424 skipping the pattern is also valid (greedy and non-greedy). The third is used 
  425 when a pattern has the quantifier {0,0}. It cannot be entirely discarded,
  426 because it may be called as a subroutine from elsewhere in the regex.
  427 
  428 A subpattern with an indefinite maximum repetition is replicated in the
  429 compiled data its minimum number of times (or once with OP_BRAZERO if the
  430 minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
  431 as appropriate.
  432 
  433 A subpattern with a bounded maximum repetition is replicated in a nested
  434 fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
  435 before each replication after the minimum, so that, for example, (abc){2,5} is
  436 compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group 
  437 has the same number.
  438 
  439 When a repeated subpattern has an unbounded upper limit, it is checked to see 
  440 whether it could match an empty string. If this is the case, the opcode in the 
  441 final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher
  442 that it needs to check for matching an empty string when it hits OP_KETRMIN or
  443 OP_KETRMAX, and if so, to break the loop.
  444 
  445 
  446 Possessive brackets
  447 -------------------
  448 
  449 When a repeated group (capturing or non-capturing) is marked as possessive by
  450 the "+" notation, e.g. (abc)++, different opcodes are used. Their names all
  451 have POS on the end, e.g. OP_BRAPOS instead of OP_BRA and OP_SCPBRPOS instead 
  452 of OP_SCBRA. The end of such a group is marked by OP_KETRPOS. If the minimum 
  453 repetition is zero, the group is preceded by OP_BRAPOSZERO.
  454 
  455 
  456 Once-only (atomic) groups
  457 -------------------------
  458 
  459 These are just like other subpatterns, but they start with the opcode
  460 OP_ONCE or OP_ONCE_NC. The former is used when there are no capturing brackets 
  461 within the atomic group; the latter when there are. The distinction is needed 
  462 for when there is a backtrack to before the group - any captures within the 
  463 group must be reset, so it is necessary to retain backtracking points inside
  464 the group even after it is complete in order to do this. When there are no 
  465 captures in an atomic group, all the backtracking can be discarded when it is 
  466 complete. This is more efficient, and also uses less stack.
  467 
  468 The check for matching an empty string in an unbounded repeat is handled
  469 entirely at runtime, so there are just these two opcodes for atomic groups.
  470 
  471 
  472 Assertions
  473 ----------
  474 
  475 Forward assertions are also just like other subpatterns, but starting with one
  476 of the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
  477 OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
  478 is OP_REVERSE, followed by a count of the number of characters to move back the
  479 pointer in the subject string. In ASCII mode, the count is a number of units,
  480 but in UTF-8/16 mode each character may occupy more than one unit; in UTF-32
  481 mode each character occupies exactly one unit. A separate count is present in
  482 each alternative of a lookbehind assertion, allowing them to have different
  483 fixed lengths.
  484 
  485 
  486 Conditional subpatterns
  487 -----------------------
  488 
  489 These are like other subpatterns, but they start with the opcode OP_COND, or
  490 OP_SCOND for one that might match an empty string in an unbounded repeat. If
  491 the condition is a back reference, this is stored at the start of the
  492 subpattern using the opcode OP_CREF followed by a count containing the
  493 reference number, provided that the reference is to a unique capturing group.
  494 If the reference was by name and there is more than one group with that name, 
  495 OP_DNCREF is used instead. It is followed by two counts: the index in the group 
  496 names table, and the number of groups with the same name.
  497 
  498 If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
  499 group x" (coded as "(?(Rx)"), the group number is stored at the start of the
  500 subpattern using the opcode OP_RREF (with a value of zero for "the whole
  501 pattern") or OP_DNRREF (with data as for OP_DNCREF). For a DEFINE condition,
  502 just the single unit OP_DEF is used (it has no associated data). Otherwise, a
  503 conditional subpattern always starts with one of the assertions.
  504 
  505 
  506 Recursion
  507 ---------
  508 
  509 Recursion either matches the current regex, or some subexpression. The opcode
  510 OP_RECURSE is followed by aLINK_SIZE value that is the offset to the starting
  511 bracket from the start of the whole pattern. From release 6.5, OP_RECURSE is
  512 automatically wrapped inside OP_ONCE brackets, because otherwise some patterns
  513 broke it. OP_RECURSE is also used for "subroutine" calls, even though they are
  514 not strictly a recursion.
  515 
  516 
  517 Callout
  518 -------
  519 
  520 OP_CALLOUT is followed by one unit of data that holds a callout number in the
  521 range 0 to 254 for manual callouts, or 255 for an automatic callout. In both 
  522 cases there follows a count giving the offset in the pattern string to the
  523 start of the following item, and another count giving the length of this item.
  524 These values make is possible for pcretest to output useful tracing information 
  525 using automatic callouts.
  526 
  527 Philip Hazel
  528 November 2013