diff options
Diffstat (limited to 'src/third_party/pcre-8.42/HACKING')
| -rw-r--r-- | src/third_party/pcre-8.42/HACKING | 528 |
1 files changed, 0 insertions, 528 deletions
diff --git a/src/third_party/pcre-8.42/HACKING b/src/third_party/pcre-8.42/HACKING deleted file mode 100644 index 691b7a14e50..00000000000 --- a/src/third_party/pcre-8.42/HACKING +++ /dev/null @@ -1,528 +0,0 @@ -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 |