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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 data strings
   53 -------------------------------
   54 
   55 From release 8.30, PCRE supports 16-bit as well as 8-bit data strings, by being 
   56 compilable in either 8-bit or 16-bit modes, or both. Thus, two different 
   57 libraries can be created. In the description that follows, the word "short" is 
   58 used for a 16-bit data quantity, and the word "unit" is used for a quantity
   59 that is a byte in 8-bit mode and a short in 16-bit mode. However, so as not to
   60 over-complicate the text, the names of PCRE functions are given in 8-bit form
   61 only.
   62 
   63 
   64 Computing the memory requirement: how it was
   65 --------------------------------------------
   66 
   67 Up to and including release 6.7, PCRE worked by running a very degenerate first
   68 pass to calculate a maximum store size, and then a second pass to do the real
   69 compile - which might use a bit less than the predicted amount of memory. The
   70 idea was that this would turn out faster than the Henry Spencer code because
   71 the first pass is degenerate and the second pass can just store stuff straight
   72 into the vector, which it knows is big enough.
   73 
   74 
   75 Computing the memory requirement: how it is
   76 -------------------------------------------
   77 
   78 By the time I was working on a potential 6.8 release, the degenerate first pass
   79 had become very complicated and hard to maintain. Indeed one of the early
   80 things I did for 6.8 was to fix Yet Another Bug in the memory computation. Then
   81 I had a flash of inspiration as to how I could run the real compile function in
   82 a "fake" mode that enables it to compute how much memory it would need, while
   83 actually only ever using a few hundred bytes of working memory, and without too
   84 many tests of the mode that might slow it down. So I refactored the compiling
   85 functions to work this way. This got rid of about 600 lines of source. It
   86 should make future maintenance and development easier. As this was such a major 
   87 change, I never released 6.8, instead upping the number to 7.0 (other quite 
   88 major changes were also present in the 7.0 release).
   89 
   90 A side effect of this work was that the previous limit of 200 on the nesting
   91 depth of parentheses was removed. However, there is a downside: pcre_compile()
   92 runs more slowly than before (30% or more, depending on the pattern) because it
   93 is doing a full analysis of the pattern. My hope was that this would not be a
   94 big issue, and in the event, nobody has commented on it.
   95 
   96 
   97 Traditional matching function
   98 -----------------------------
   99 
  100 The "traditional", and original, matching function is called pcre_exec(), and 
  101 it implements an NFA algorithm, similar to the original Henry Spencer algorithm 
  102 and the way that Perl works. This is not surprising, since it is intended to be
  103 as compatible with Perl as possible. This is the function most users of PCRE
  104 will use most of the time. From release 8.20, if PCRE is compiled with 
  105 just-in-time (JIT) support, and studying a compiled pattern with JIT is 
  106 successful, the JIT code is run instead of the normal pcre_exec() code, but the 
  107 result is the same.
  108 
  109 
  110 Supplementary matching function
  111 -------------------------------
  112 
  113 From PCRE 6.0, there is also a supplementary matching function called 
  114 pcre_dfa_exec(). This implements a DFA matching algorithm that searches 
  115 simultaneously for all possible matches that start at one point in the subject 
  116 string. (Going back to my roots: see Historical Note 1 above.) This function 
  117 intreprets the same compiled pattern data as pcre_exec(); however, not all the 
  118 facilities are available, and those that are do not always work in quite the 
  119 same way. See the user documentation for details.
  120 
  121 The algorithm that is used for pcre_dfa_exec() is not a traditional FSM, 
  122 because it may have a number of states active at one time. More work would be 
  123 needed at compile time to produce a traditional FSM where only one state is 
  124 ever active at once. I believe some other regex matchers work this way.
  125 
  126 
  127 Changeable options
  128 ------------------
  129 
  130 The /i, /m, or /s options (PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL) may
  131 change in the middle of patterns. From PCRE 8.13, their processing is handled
  132 entirely at compile time by generating different opcodes for the different
  133 settings. The runtime functions do not need to keep track of an options state 
  134 any more.
  135 
  136 
  137 Format of compiled patterns
  138 ---------------------------
  139 
  140 The compiled form of a pattern is a vector of units (bytes in 8-bit mode, or
  141 shorts in 16-bit mode), containing items of variable length. The first unit in
  142 an item contains an opcode, and the length of the item is either implicit in
  143 the opcode or contained in the data that follows it.
  144 
  145 In many cases listed below, LINK_SIZE data values are specified for offsets
  146 within the compiled pattern. LINK_SIZE always specifies a number of bytes. The
  147 default value for LINK_SIZE is 2, but PCRE can be compiled to use 3-byte or
  148 4-byte values for these offsets, although this impairs the performance. (3-byte
  149 LINK_SIZE values are available only in 8-bit mode.) Specifing a LINK_SIZE
  150 larger than 2 is necessary only when patterns whose compiled length is greater
  151 than 64K are going to be processed. In this description, we assume the "normal"
  152 compilation options. Data values that are counts (e.g. for quantifiers) are
  153 always just two bytes long (one short in 16-bit mode).
  154 
  155 Opcodes with no following data
  156 ------------------------------
  157 
  158 These items are all just one unit long
  159 
  160   OP_END                 end of pattern
  161   OP_ANY                 match any one character other than newline
  162   OP_ALLANY              match any one character, including newline
  163   OP_ANYBYTE             match any single byte, even in UTF-8 mode
  164   OP_SOD                 match start of data: \A
  165   OP_SOM,                start of match (subject + offset): \G
  166   OP_SET_SOM,            set start of match (\K) 
  167   OP_CIRC                ^ (start of data)
  168   OP_CIRCM               ^ multiline mode (start of data or after newline)
  169   OP_NOT_WORD_BOUNDARY   \W
  170   OP_WORD_BOUNDARY       \w
  171   OP_NOT_DIGIT           \D
  172   OP_DIGIT               \d
  173   OP_NOT_HSPACE          \H
  174   OP_HSPACE              \h  
  175   OP_NOT_WHITESPACE      \S
  176   OP_WHITESPACE          \s
  177   OP_NOT_VSPACE          \V
  178   OP_VSPACE              \v  
  179   OP_NOT_WORDCHAR        \W
  180   OP_WORDCHAR            \w
  181   OP_EODN                match end of data or \n at end: \Z
  182   OP_EOD                 match end of data: \z
  183   OP_DOLL                $ (end of data, or before final newline)
  184   OP_DOLLM               $ multiline mode (end of data or before newline)
  185   OP_EXTUNI              match an extended Unicode character 
  186   OP_ANYNL               match any Unicode newline sequence 
  187   
  188   OP_ACCEPT              ) These are Perl 5.10's "backtracking control   
  189   OP_COMMIT              ) verbs". If OP_ACCEPT is inside capturing
  190   OP_FAIL                ) parentheses, it may be preceded by one or more
  191   OP_PRUNE               ) OP_CLOSE, followed by a 2-byte number,
  192   OP_SKIP                ) indicating which parentheses must be closed.
  193   
  194 
  195 Backtracking control verbs with (optional) data
  196 -----------------------------------------------
  197 
  198 (*THEN) without an argument generates the opcode OP_THEN and no following data.
  199 OP_MARK is followed by the mark name, preceded by a one-unit length, and
  200 followed by a binary zero. For (*PRUNE), (*SKIP), and (*THEN) with arguments,
  201 the opcodes OP_PRUNE_ARG, OP_SKIP_ARG, and OP_THEN_ARG are used, with the name
  202 following in the same format.
  203   
  204 
  205 Matching literal characters
  206 ---------------------------
  207 
  208 The OP_CHAR opcode is followed by a single character that is to be matched 
  209 casefully. For caseless matching, OP_CHARI is used. In UTF-8 or UTF-16 modes,
  210 the character may be more than one unit long.
  211 
  212 
  213 Repeating single characters
  214 ---------------------------
  215 
  216 The common repeats (*, +, ?), when applied to a single character, use the
  217 following opcodes, which come in caseful and caseless versions:
  218 
  219   Caseful         Caseless
  220   OP_STAR         OP_STARI      
  221   OP_MINSTAR      OP_MINSTARI   
  222   OP_POSSTAR      OP_POSSTARI   
  223   OP_PLUS         OP_PLUSI      
  224   OP_MINPLUS      OP_MINPLUSI   
  225   OP_POSPLUS      OP_POSPLUSI   
  226   OP_QUERY        OP_QUERYI     
  227   OP_MINQUERY     OP_MINQUERYI  
  228   OP_POSQUERY     OP_POSQUERYI  
  229 
  230 Each opcode is followed by the character that is to be repeated. In ASCII mode,
  231 these are two-unit items; in UTF-8 or UTF-16 modes, the length is variable.
  232 Those with "MIN" in their names are the minimizing versions. Those with "POS"
  233 in their names are possessive versions. Other repeats make use of these
  234 opcodes:
  235 
  236   Caseful         Caseless
  237   OP_UPTO         OP_UPTOI    
  238   OP_MINUPTO      OP_MINUPTOI 
  239   OP_POSUPTO      OP_POSUPTOI 
  240   OP_EXACT        OP_EXACTI   
  241 
  242 Each of these is followed by a two-byte (one short) count (most significant
  243 byte first in 8-bit mode) and then the repeated character. OP_UPTO matches from
  244 0 to the given number. A repeat with a non-zero minimum and a fixed maximum is
  245 coded as an OP_EXACT followed by an OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).
  246 
  247 
  248 Repeating character types
  249 -------------------------
  250 
  251 Repeats of things like \d are done exactly as for single characters, except
  252 that instead of a character, the opcode for the type is stored in the data
  253 unit. The opcodes are:
  254 
  255   OP_TYPESTAR
  256   OP_TYPEMINSTAR
  257   OP_TYPEPOSSTAR 
  258   OP_TYPEPLUS
  259   OP_TYPEMINPLUS
  260   OP_TYPEPOSPLUS 
  261   OP_TYPEQUERY
  262   OP_TYPEMINQUERY
  263   OP_TYPEPOSQUERY 
  264   OP_TYPEUPTO
  265   OP_TYPEMINUPTO
  266   OP_TYPEPOSUPTO 
  267   OP_TYPEEXACT
  268 
  269 
  270 Match by Unicode property
  271 -------------------------
  272 
  273 OP_PROP and OP_NOTPROP are used for positive and negative matches of a 
  274 character by testing its Unicode property (the \p and \P escape sequences).
  275 Each is followed by two units that encode the desired property as a type and a
  276 value.
  277 
  278 Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by
  279 three units: OP_PROP or OP_NOTPROP, and then the desired property type and
  280 value.
  281 
  282 
  283 Character classes
  284 -----------------
  285 
  286 If there is only one character in the class, OP_CHAR or OP_CHARI is used for a
  287 positive class, and OP_NOT or OP_NOTI for a negative one (that is, for
  288 something like [^a]). However, OP_NOT[I] can be used only with single-unit
  289 characters, so in UTF-8 (UTF-16) mode, the use of OP_NOT[I] applies only to
  290 characters whose code points are no greater than 127 (0xffff).
  291 
  292 Another set of 13 repeating opcodes (called OP_NOTSTAR etc.) are used for
  293 repeated, negated, single-character classes. The normal single-character
  294 opcodes (OP_STAR, etc.) are used for repeated positive single-character
  295 classes.
  296 
  297 When there is more than one character in a class and all the characters are
  298 less than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a
  299 negative one. In either case, the opcode is followed by a 32-byte (16-short)
  300 bit map containing a 1 bit for every character that is acceptable. The bits are
  301 counted from the least significant end of each unit. In caseless mode, bits for
  302 both cases are set.
  303 
  304 The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8/16 mode,
  305 subject characters with values greater than 255 can be handled correctly. For
  306 OP_CLASS they do not match, whereas for OP_NCLASS they do.
  307 
  308 For classes containing characters with values greater than 255, OP_XCLASS is
  309 used. It optionally uses a bit map (if any characters lie within it), followed
  310 by a list of pairs (for a range) and single characters. In caseless mode, both
  311 cases are explicitly listed. There is a flag character than indicates whether
  312 it is a positive or a negative class.
  313 
  314 
  315 Back references
  316 ---------------
  317 
  318 OP_REF (caseful) or OP_REFI (caseless) is followed by two bytes (one short)
  319 containing the reference number.
  320 
  321 
  322 Repeating character classes and back references
  323 -----------------------------------------------
  324 
  325 Single-character classes are handled specially (see above). This section
  326 applies to OP_CLASS and OP_REF[I]. In both cases, the repeat information
  327 follows the base item. The matching code looks at the following opcode to see
  328 if it is one of
  329 
  330   OP_CRSTAR
  331   OP_CRMINSTAR
  332   OP_CRPLUS
  333   OP_CRMINPLUS
  334   OP_CRQUERY
  335   OP_CRMINQUERY
  336   OP_CRRANGE
  337   OP_CRMINRANGE
  338 
  339 All but the last two are just single-unit items. The others are followed by
  340 four bytes (two shorts) of data, comprising the minimum and maximum repeat
  341 counts. There are no special possessive opcodes for these repeats; a possessive
  342 repeat is compiled into an atomic group.
  343 
  344 
  345 Brackets and alternation
  346 ------------------------
  347 
  348 A pair of non-capturing (round) brackets is wrapped round each expression at
  349 compile time, so alternation always happens in the context of brackets.
  350 
  351 [Note for North Americans: "bracket" to some English speakers, including
  352 myself, can be round, square, curly, or pointy. Hence this usage rather than 
  353 "parentheses".]
  354 
  355 Non-capturing brackets use the opcode OP_BRA. Originally PCRE was limited to 99
  356 capturing brackets and it used a different opcode for each one. From release
  357 3.5, the limit was removed by putting the bracket number into the data for
  358 higher-numbered brackets. From release 7.0 all capturing brackets are handled
  359 this way, using the single opcode OP_CBRA.
  360 
  361 A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
  362 next alternative OP_ALT or, if there aren't any branches, to the matching
  363 OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
  364 the next one, or to the OP_KET opcode. For capturing brackets, the bracket 
  365 number immediately follows the offset, always as a 2-byte (one short) item.
  366 
  367 OP_KET is used for subpatterns that do not repeat indefinitely, and
  368 OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
  369 maximally respectively (see below for possessive repetitions). All three are
  370 followed by LINK_SIZE bytes giving (as a positive number) the offset back to
  371 the matching bracket opcode.
  372 
  373 If a subpattern is quantified such that it is permitted to match zero times, it
  374 is preceded by one of OP_BRAZERO, OP_BRAMINZERO, or OP_SKIPZERO. These are
  375 single-unit opcodes that tell the matcher that skipping the following
  376 subpattern entirely is a valid branch. In the case of the first two, not 
  377 skipping the pattern is also valid (greedy and non-greedy). The third is used 
  378 when a pattern has the quantifier {0,0}. It cannot be entirely discarded, 
  379 because it may be called as a subroutine from elsewhere in the regex.
  380 
  381 A subpattern with an indefinite maximum repetition is replicated in the
  382 compiled data its minimum number of times (or once with OP_BRAZERO if the
  383 minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
  384 as appropriate.
  385 
  386 A subpattern with a bounded maximum repetition is replicated in a nested
  387 fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
  388 before each replication after the minimum, so that, for example, (abc){2,5} is
  389 compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group 
  390 has the same number.
  391 
  392 When a repeated subpattern has an unbounded upper limit, it is checked to see 
  393 whether it could match an empty string. If this is the case, the opcode in the 
  394 final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher
  395 that it needs to check for matching an empty string when it hits OP_KETRMIN or
  396 OP_KETRMAX, and if so, to break the loop.
  397 
  398 Possessive brackets
  399 -------------------
  400 
  401 When a repeated group (capturing or non-capturing) is marked as possessive by
  402 the "+" notation, e.g. (abc)++, different opcodes are used. Their names all
  403 have POS on the end, e.g. OP_BRAPOS instead of OP_BRA and OP_SCPBRPOS instead 
  404 of OP_SCBRA. The end of such a group is marked by OP_KETRPOS. If the minimum 
  405 repetition is zero, the group is preceded by OP_BRAPOSZERO.
  406 
  407 
  408 Assertions
  409 ----------
  410 
  411 Forward assertions are just like other subpatterns, but starting with one of
  412 the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
  413 OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
  414 is OP_REVERSE, followed by a two byte (one short) count of the number of
  415 characters to move back the pointer in the subject string. In ASCII mode, the 
  416 count is a number of units, but in UTF-8/16 mode each character may occupy more
  417 than one unit. A separate count is present in each alternative of a lookbehind
  418 assertion, allowing them to have different fixed lengths.
  419 
  420 
  421 Once-only (atomic) subpatterns
  422 ------------------------------
  423 
  424 These are also just like other subpatterns, but they start with the opcode
  425 OP_ONCE. The check for matching an empty string in an unbounded repeat is 
  426 handled entirely at runtime, so there is just this one opcode.
  427 
  428 
  429 Conditional subpatterns
  430 -----------------------
  431 
  432 These are like other subpatterns, but they start with the opcode OP_COND, or
  433 OP_SCOND for one that might match an empty string in an unbounded repeat. If
  434 the condition is a back reference, this is stored at the start of the
  435 subpattern using the opcode OP_CREF followed by two bytes (one short)
  436 containing the reference number. OP_NCREF is used instead if the reference was
  437 generated by name (so that the runtime code knows to check for duplicate
  438 names).
  439 
  440 If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
  441 group x" (coded as "(?(Rx)"), the group number is stored at the start of the
  442 subpattern using the opcode OP_RREF or OP_NRREF (cf OP_NCREF), and a value of
  443 zero for "the whole pattern". For a DEFINE condition, just the single unit
  444 OP_DEF is used (it has no associated data). Otherwise, a conditional subpattern
  445 always starts with one of the assertions.
  446 
  447 
  448 Recursion
  449 ---------
  450 
  451 Recursion either matches the current regex, or some subexpression. The opcode
  452 OP_RECURSE is followed by an value which is the offset to the starting bracket
  453 from the start of the whole pattern. From release 6.5, OP_RECURSE is 
  454 automatically wrapped inside OP_ONCE brackets (because otherwise some patterns 
  455 broke it). OP_RECURSE is also used for "subroutine" calls, even though they 
  456 are not strictly a recursion.
  457 
  458 
  459 Callout
  460 -------
  461 
  462 OP_CALLOUT is followed by one unit of data that holds a callout number in the
  463 range 0 to 254 for manual callouts, or 255 for an automatic callout. In both 
  464 cases there follows a two-byte (one short) value giving the offset in the
  465 pattern to the start of the following item, and another two-byte (one short)
  466 item giving the length of the next item.
  467 
  468 
  469 Philip Hazel
  470 December 2011