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@@ -0,0 +1,528 @@ +Technical Notes about PCRE +-------------------------- + +These are very rough technical notes that record potentially useful information +about PCRE internals. For information about testing PCRE, see the pcretest +documentation and the comment at the head of the RunTest file. + + +Historical note 1 +----------------- + +Many years ago I implemented some regular expression functions to an algorithm +suggested by Martin Richards. These were not Unix-like in form, and were quite +restricted in what they could do by comparison with Perl. The interesting part +about the algorithm was that the amount of space required to hold the compiled +form of an expression was known in advance. The code to apply an expression did +not operate by backtracking, as the original Henry Spencer code and current +Perl code does, but instead checked all possibilities simultaneously by keeping +a list of current states and checking all of them as it advanced through the +subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA +algorithm", though it was not a traditional Finite State Machine (FSM). When +the pattern was all used up, all remaining states were possible matches, and +the one matching the longest subset of the subject string was chosen. This did +not necessarily maximize the individual wild portions of the pattern, as is +expected in Unix and Perl-style regular expressions. + + +Historical note 2 +----------------- + +By contrast, the code originally written by Henry Spencer (which was +subsequently heavily modified for Perl) compiles the expression twice: once in +a dummy mode in order to find out how much store will be needed, and then for +real. (The Perl version probably doesn't do this any more; I'm talking about +the original library.) The execution function operates by backtracking and +maximizing (or, optionally, minimizing in Perl) the amount of the subject that +matches individual wild portions of the pattern. This is an "NFA algorithm" in +Friedl's terminology. + + +OK, here's the real stuff +------------------------- + +For the set of functions that form the "basic" PCRE library (which are +unrelated to those mentioned above), I tried at first to invent an algorithm +that used an amount of store bounded by a multiple of the number of characters +in the pattern, to save on compiling time. However, because of the greater +complexity in Perl regular expressions, I couldn't do this. In any case, a +first pass through the pattern is helpful for other reasons. + + +Support for 16-bit and 32-bit data strings +------------------------------------------- + +From release 8.30, PCRE supports 16-bit as well as 8-bit data strings; and from +release 8.32, PCRE supports 32-bit data strings. The library can be compiled +in any combination of 8-bit, 16-bit or 32-bit modes, creating up to three +different libraries. In the description that follows, the word "short" is used +for a 16-bit data quantity, and the word "unit" is used for a quantity that is +a byte in 8-bit mode, a short in 16-bit mode and a 32-bit word in 32-bit mode. +However, so as not to over-complicate the text, the names of PCRE functions are +given in 8-bit form only. + + +Computing the memory requirement: how it was +-------------------------------------------- + +Up to and including release 6.7, PCRE worked by running a very degenerate first +pass to calculate a maximum store size, and then a second pass to do the real +compile - which might use a bit less than the predicted amount of memory. The +idea was that this would turn out faster than the Henry Spencer code because +the first pass is degenerate and the second pass can just store stuff straight +into the vector, which it knows is big enough. + + +Computing the memory requirement: how it is +------------------------------------------- + +By the time I was working on a potential 6.8 release, the degenerate first pass +had become very complicated and hard to maintain. Indeed one of the early +things I did for 6.8 was to fix Yet Another Bug in the memory computation. Then +I had a flash of inspiration as to how I could run the real compile function in +a "fake" mode that enables it to compute how much memory it would need, while +actually only ever using a few hundred bytes of working memory, and without too +many tests of the mode that might slow it down. So I refactored the compiling +functions to work this way. This got rid of about 600 lines of source. It +should make future maintenance and development easier. As this was such a major +change, I never released 6.8, instead upping the number to 7.0 (other quite +major changes were also present in the 7.0 release). + +A side effect of this work was that the previous limit of 200 on the nesting +depth of parentheses was removed. However, there is a downside: pcre_compile() +runs more slowly than before (30% or more, depending on the pattern) because it +is doing a full analysis of the pattern. My hope was that this would not be a +big issue, and in the event, nobody has commented on it. + +At release 8.34, a limit on the nesting depth of parentheses was re-introduced +(default 250, settable at build time) so as to put a limit on the amount of +system stack used by pcre_compile(). This is a safety feature for environments +with small stacks where the patterns are provided by users. + + +Traditional matching function +----------------------------- + +The "traditional", and original, matching function is called pcre_exec(), and +it implements an NFA algorithm, similar to the original Henry Spencer algorithm +and the way that Perl works. This is not surprising, since it is intended to be +as compatible with Perl as possible. This is the function most users of PCRE +will use most of the time. From release 8.20, if PCRE is compiled with +just-in-time (JIT) support, and studying a compiled pattern with JIT is +successful, the JIT code is run instead of the normal pcre_exec() code, but the +result is the same. + + +Supplementary matching function +------------------------------- + +From PCRE 6.0, there is also a supplementary matching function called +pcre_dfa_exec(). This implements a DFA matching algorithm that searches +simultaneously for all possible matches that start at one point in the subject +string. (Going back to my roots: see Historical Note 1 above.) This function +intreprets the same compiled pattern data as pcre_exec(); however, not all the +facilities are available, and those that are do not always work in quite the +same way. See the user documentation for details. + +The algorithm that is used for pcre_dfa_exec() is not a traditional FSM, +because it may have a number of states active at one time. More work would be +needed at compile time to produce a traditional FSM where only one state is +ever active at once. I believe some other regex matchers work this way. JIT +support is not available for this kind of matching. + + +Changeable options +------------------ + +The /i, /m, or /s options (PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and some +others) may change in the middle of patterns. From PCRE 8.13, their processing +is handled entirely at compile time by generating different opcodes for the +different settings. The runtime functions do not need to keep track of an +options state any more. + + +Format of compiled patterns +--------------------------- + +The compiled form of a pattern is a vector of unsigned units (bytes in 8-bit +mode, shorts in 16-bit mode, 32-bit words in 32-bit mode), containing items of +variable length. The first unit in an item contains an opcode, and the length +of the item is either implicit in the opcode or contained in the data that +follows it. + +In many cases listed below, LINK_SIZE data values are specified for offsets +within the compiled pattern. LINK_SIZE always specifies a number of bytes. The +default value for LINK_SIZE is 2, but PCRE can be compiled to use 3-byte or +4-byte values for these offsets, although this impairs the performance. (3-byte +LINK_SIZE values are available only in 8-bit mode.) Specifing a LINK_SIZE +larger than 2 is necessary only when patterns whose compiled length is greater +than 64K are going to be processed. In this description, we assume the "normal" +compilation options. Data values that are counts (e.g. quantifiers) are two +bytes long in 8-bit mode (most significant byte first), or one unit in 16-bit +and 32-bit modes. + + +Opcodes with no following data +------------------------------ + +These items are all just one unit long + + OP_END end of pattern + OP_ANY match any one character other than newline + OP_ALLANY match any one character, including newline + OP_ANYBYTE match any single unit, even in UTF-8/16 mode + OP_SOD match start of data: \A + OP_SOM, start of match (subject + offset): \G + OP_SET_SOM, set start of match (\K) + OP_CIRC ^ (start of data) + OP_CIRCM ^ multiline mode (start of data or after newline) + OP_NOT_WORD_BOUNDARY \W + OP_WORD_BOUNDARY \w + OP_NOT_DIGIT \D + OP_DIGIT \d + OP_NOT_HSPACE \H + OP_HSPACE \h + OP_NOT_WHITESPACE \S + OP_WHITESPACE \s + OP_NOT_VSPACE \V + OP_VSPACE \v + OP_NOT_WORDCHAR \W + OP_WORDCHAR \w + OP_EODN match end of data or newline at end: \Z + OP_EOD match end of data: \z + OP_DOLL $ (end of data, or before final newline) + OP_DOLLM $ multiline mode (end of data or before newline) + OP_EXTUNI match an extended Unicode grapheme cluster + OP_ANYNL match any Unicode newline sequence + + OP_ASSERT_ACCEPT ) + OP_ACCEPT ) These are Perl 5.10's "backtracking control + OP_COMMIT ) verbs". If OP_ACCEPT is inside capturing + OP_FAIL ) parentheses, it may be preceded by one or more + OP_PRUNE ) OP_CLOSE, each followed by a count that + OP_SKIP ) indicates which parentheses must be closed. + OP_THEN ) + +OP_ASSERT_ACCEPT is used when (*ACCEPT) is encountered within an assertion. +This ends the assertion, not the entire pattern match. + + +Backtracking control verbs with optional data +--------------------------------------------- + +(*THEN) without an argument generates the opcode OP_THEN and no following data. +OP_MARK is followed by the mark name, preceded by a one-unit length, and +followed by a binary zero. For (*PRUNE), (*SKIP), and (*THEN) with arguments, +the opcodes OP_PRUNE_ARG, OP_SKIP_ARG, and OP_THEN_ARG are used, with the name +following in the same format as OP_MARK. + + +Matching literal characters +--------------------------- + +The OP_CHAR opcode is followed by a single character that is to be matched +casefully. For caseless matching, OP_CHARI is used. In UTF-8 or UTF-16 modes, +the character may be more than one unit long. In UTF-32 mode, characters +are always exactly one unit long. + +If there is only one character in a character class, OP_CHAR or OP_CHARI is +used for a positive class, and OP_NOT or OP_NOTI for a negative one (that is, +for something like [^a]). + + +Repeating single characters +--------------------------- + +The common repeats (*, +, ?), when applied to a single character, use the +following opcodes, which come in caseful and caseless versions: + + Caseful Caseless + OP_STAR OP_STARI + OP_MINSTAR OP_MINSTARI + OP_POSSTAR OP_POSSTARI + OP_PLUS OP_PLUSI + OP_MINPLUS OP_MINPLUSI + OP_POSPLUS OP_POSPLUSI + OP_QUERY OP_QUERYI + OP_MINQUERY OP_MINQUERYI + OP_POSQUERY OP_POSQUERYI + +Each opcode is followed by the character that is to be repeated. In ASCII mode, +these are two-unit items; in UTF-8 or UTF-16 modes, the length is variable; in +UTF-32 mode these are one-unit items. Those with "MIN" in their names are the +minimizing versions. Those with "POS" in their names are possessive versions. +Other repeats make use of these opcodes: + + Caseful Caseless + OP_UPTO OP_UPTOI + OP_MINUPTO OP_MINUPTOI + OP_POSUPTO OP_POSUPTOI + OP_EXACT OP_EXACTI + +Each of these is followed by a count and then the repeated character. OP_UPTO +matches from 0 to the given number. A repeat with a non-zero minimum and a +fixed maximum is coded as an OP_EXACT followed by an OP_UPTO (or OP_MINUPTO or +OPT_POSUPTO). + +Another set of matching repeating opcodes (called OP_NOTSTAR, OP_NOTSTARI, +etc.) are used for repeated, negated, single-character classes such as [^a]*. +The normal single-character opcodes (OP_STAR, etc.) are used for repeated +positive single-character classes. + + +Repeating character types +------------------------- + +Repeats of things like \d are done exactly as for single characters, except +that instead of a character, the opcode for the type is stored in the data +unit. The opcodes are: + + OP_TYPESTAR + OP_TYPEMINSTAR + OP_TYPEPOSSTAR + OP_TYPEPLUS + OP_TYPEMINPLUS + OP_TYPEPOSPLUS + OP_TYPEQUERY + OP_TYPEMINQUERY + OP_TYPEPOSQUERY + OP_TYPEUPTO + OP_TYPEMINUPTO + OP_TYPEPOSUPTO + OP_TYPEEXACT + + +Match by Unicode property +------------------------- + +OP_PROP and OP_NOTPROP are used for positive and negative matches of a +character by testing its Unicode property (the \p and \P escape sequences). +Each is followed by two units that encode the desired property as a type and a +value. The types are a set of #defines of the form PT_xxx, and the values are +enumerations of the form ucp_xx, defined in the ucp.h source file. The value is +relevant only for PT_GC (General Category), PT_PC (Particular Category), and +PT_SC (Script). + +Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by +three units: OP_PROP or OP_NOTPROP, and then the desired property type and +value. + + +Character classes +----------------- + +If there is only one character in a class, OP_CHAR or OP_CHARI is used for a +positive class, and OP_NOT or OP_NOTI for a negative one (that is, for +something like [^a]). + +A set of repeating opcodes (called OP_NOTSTAR etc.) are used for repeated, +negated, single-character classes. The normal single-character opcodes +(OP_STAR, etc.) are used for repeated positive single-character classes. + +When there is more than one character in a class, and all the code points are +less than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a +negative one. In either case, the opcode is followed by a 32-byte (16-short, +8-word) bit map containing a 1 bit for every character that is acceptable. The +bits are counted from the least significant end of each unit. In caseless mode, +bits for both cases are set. + +The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8/16/32 +mode, subject characters with values greater than 255 can be handled correctly. +For OP_CLASS they do not match, whereas for OP_NCLASS they do. + +For classes containing characters with values greater than 255 or that contain +\p or \P, OP_XCLASS is used. It optionally uses a bit map if any code points +are less than 256, followed by a list of pairs (for a range) and single +characters. In caseless mode, both cases are explicitly listed. + +OP_XCLASS is followed by a unit containing flag bits: XCL_NOT indicates that +this is a negative class, and XCL_MAP indicates that a bit map is present. +There follows the bit map, if XCL_MAP is set, and then a sequence of items +coded as follows: + + XCL_END marks the end of the list + XCL_SINGLE one character follows + XCL_RANGE two characters follow + XCL_PROP a Unicode property (type, value) follows + XCL_NOTPROP a Unicode property (type, value) follows + +If a range starts with a code point less than 256 and ends with one greater +than 256, an XCL_RANGE item is used, without setting any bits in the bit map. +This means that if no other items in the class set bits in the map, a map is +not needed. + + +Back references +--------------- + +OP_REF (caseful) or OP_REFI (caseless) is followed by a count containing the +reference number if the reference is to a unique capturing group (either by +number or by name). When named groups are used, there may be more than one +group with the same name. In this case, a reference by name generates OP_DNREF +or OP_DNREFI. These are followed by two counts: the index (not the byte offset) +in the group name table of the first entry for the requred name, followed by +the number of groups with the same name. + + +Repeating character classes and back references +----------------------------------------------- + +Single-character classes are handled specially (see above). This section +applies to other classes and also to back references. In both cases, the repeat +information follows the base item. The matching code looks at the following +opcode to see if it is one of + + OP_CRSTAR + OP_CRMINSTAR + OP_CRPOSSTAR + OP_CRPLUS + OP_CRMINPLUS + OP_CRPOSPLUS + OP_CRQUERY + OP_CRMINQUERY + OP_CRPOSQUERY + OP_CRRANGE + OP_CRMINRANGE + OP_CRPOSRANGE + +All but the last three are single-unit items, with no data. The others are +followed by the minimum and maximum repeat counts. + + +Brackets and alternation +------------------------ + +A pair of non-capturing round brackets is wrapped round each expression at +compile time, so alternation always happens in the context of brackets. + +[Note for North Americans: "bracket" to some English speakers, including +myself, can be round, square, curly, or pointy. Hence this usage rather than +"parentheses".] + +Non-capturing brackets use the opcode OP_BRA. Originally PCRE was limited to 99 +capturing brackets and it used a different opcode for each one. From release +3.5, the limit was removed by putting the bracket number into the data for +higher-numbered brackets. From release 7.0 all capturing brackets are handled +this way, using the single opcode OP_CBRA. + +A bracket opcode is followed by LINK_SIZE bytes which give the offset to the +next alternative OP_ALT or, if there aren't any branches, to the matching +OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to +the next one, or to the OP_KET opcode. For capturing brackets, the bracket +number is a count that immediately follows the offset. + +OP_KET is used for subpatterns that do not repeat indefinitely, and OP_KETRMIN +and OP_KETRMAX are used for indefinite repetitions, minimally or maximally +respectively (see below for possessive repetitions). All three are followed by +LINK_SIZE bytes giving (as a positive number) the offset back to the matching +bracket opcode. + +If a subpattern is quantified such that it is permitted to match zero times, it +is preceded by one of OP_BRAZERO, OP_BRAMINZERO, or OP_SKIPZERO. These are +single-unit opcodes that tell the matcher that skipping the following +subpattern entirely is a valid branch. In the case of the first two, not +skipping the pattern is also valid (greedy and non-greedy). The third is used +when a pattern has the quantifier {0,0}. It cannot be entirely discarded, +because it may be called as a subroutine from elsewhere in the regex. + +A subpattern with an indefinite maximum repetition is replicated in the +compiled data its minimum number of times (or once with OP_BRAZERO if the +minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX +as appropriate. + +A subpattern with a bounded maximum repetition is replicated in a nested +fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO +before each replication after the minimum, so that, for example, (abc){2,5} is +compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group +has the same number. + +When a repeated subpattern has an unbounded upper limit, it is checked to see +whether it could match an empty string. If this is the case, the opcode in the +final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher +that it needs to check for matching an empty string when it hits OP_KETRMIN or +OP_KETRMAX, and if so, to break the loop. + + +Possessive brackets +------------------- + +When a repeated group (capturing or non-capturing) is marked as possessive by +the "+" notation, e.g. (abc)++, different opcodes are used. Their names all +have POS on the end, e.g. OP_BRAPOS instead of OP_BRA and OP_SCPBRPOS instead +of OP_SCBRA. The end of such a group is marked by OP_KETRPOS. If the minimum +repetition is zero, the group is preceded by OP_BRAPOSZERO. + + +Once-only (atomic) groups +------------------------- + +These are just like other subpatterns, but they start with the opcode +OP_ONCE or OP_ONCE_NC. The former is used when there are no capturing brackets +within the atomic group; the latter when there are. The distinction is needed +for when there is a backtrack to before the group - any captures within the +group must be reset, so it is necessary to retain backtracking points inside +the group even after it is complete in order to do this. When there are no +captures in an atomic group, all the backtracking can be discarded when it is +complete. This is more efficient, and also uses less stack. + +The check for matching an empty string in an unbounded repeat is handled +entirely at runtime, so there are just these two opcodes for atomic groups. + + +Assertions +---------- + +Forward assertions are also just like other subpatterns, but starting with one +of the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes +OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion +is OP_REVERSE, followed by a count of the number of characters to move back the +pointer in the subject string. In ASCII mode, the count is a number of units, +but in UTF-8/16 mode each character may occupy more than one unit; in UTF-32 +mode each character occupies exactly one unit. A separate count is present in +each alternative of a lookbehind assertion, allowing them to have different +fixed lengths. + + +Conditional subpatterns +----------------------- + +These are like other subpatterns, but they start with the opcode OP_COND, or +OP_SCOND for one that might match an empty string in an unbounded repeat. If +the condition is a back reference, this is stored at the start of the +subpattern using the opcode OP_CREF followed by a count containing the +reference number, provided that the reference is to a unique capturing group. +If the reference was by name and there is more than one group with that name, +OP_DNCREF is used instead. It is followed by two counts: the index in the group +names table, and the number of groups with the same name. + +If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of +group x" (coded as "(?(Rx)"), the group number is stored at the start of the +subpattern using the opcode OP_RREF (with a value of zero for "the whole +pattern") or OP_DNRREF (with data as for OP_DNCREF). For a DEFINE condition, +just the single unit OP_DEF is used (it has no associated data). Otherwise, a +conditional subpattern always starts with one of the assertions. + + +Recursion +--------- + +Recursion either matches the current regex, or some subexpression. The opcode +OP_RECURSE is followed by aLINK_SIZE value that is the offset to the starting +bracket from the start of the whole pattern. From release 6.5, OP_RECURSE is +automatically wrapped inside OP_ONCE brackets, because otherwise some patterns +broke it. OP_RECURSE is also used for "subroutine" calls, even though they are +not strictly a recursion. + + +Callout +------- + +OP_CALLOUT is followed by one unit of data that holds a callout number in the +range 0 to 254 for manual callouts, or 255 for an automatic callout. In both +cases there follows a count giving the offset in the pattern string to the +start of the following item, and another count giving the length of this item. +These values make is possible for pcretest to output useful tracing information +using automatic callouts. + +Philip Hazel +November 2013 |