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Diffstat (limited to 'ext/pcre/pcrelib/doc/Tech.Notes')
| -rw-r--r-- | ext/pcre/pcrelib/doc/Tech.Notes | 144 |
1 files changed, 99 insertions, 45 deletions
diff --git a/ext/pcre/pcrelib/doc/Tech.Notes b/ext/pcre/pcrelib/doc/Tech.Notes index 21dbe1f9b5..c75b3e8a5d 100644 --- a/ext/pcre/pcrelib/doc/Tech.Notes +++ b/ext/pcre/pcrelib/doc/Tech.Notes @@ -16,10 +16,11 @@ 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". 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. +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 ----------------- @@ -41,14 +42,38 @@ 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 needed, for a number of reasons. PCRE works -by running a very degenerate first pass to calculate a maximum store size, and -then a second pass to do the real compile - which may use a bit less than the -predicted amount of store. The idea is that this is going to turn out faster -because the first pass is degenerate and the second pass can just store stuff -straight into the vector, which it knows is big enough. It does make the -compiling functions bigger, of course, but they have become quite big anyway to -handle all the Perl stuff. +first pass through the pattern is helpful for other reasons. + +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 re-factored 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 are also present in the 7.0 release). + +A side effect of this work is 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 is that this is not a big +issue. Traditional matching function ----------------------------- @@ -70,6 +95,12 @@ 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. + + Format of compiled patterns --------------------------- @@ -79,10 +110,12 @@ item is either implicit in the opcode or contained in the data bytes that follow it. In many cases below "two-byte" data values are specified. This is in fact just -a default. PCRE can be compiled to use 3-byte or 4-byte values (impairing the +a default when the number is an offset within the compiled pattern. PCRE can be +compiled to use 3-byte or 4-byte values for these offsets (impairing the performance). This 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. +greater than 64K are going to be processed. In this description, we assume the +"normal" compilation options. "Two-byte" data values that are counts (e.g. for +quantifiers) are always just two bytes. A list of all the opcodes follows: @@ -109,6 +142,7 @@ These items are all just one byte long OP_EOD match end of data: \z OP_DOLL $ (end of data, or before \n in multiline) OP_EXTUNI match an extended Unicode character + OP_ANYNL match any Unicode newline sequence Repeating single characters @@ -119,23 +153,28 @@ following opcodes: OP_STAR OP_MINSTAR + OP_POSSTAR OP_PLUS OP_MINPLUS + OP_POSPLUS OP_QUERY OP_MINQUERY + OP_POSQUERY In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable. -Those with "MIN" in their name are the minimizing versions. Each is followed by -the character that is to be repeated. Other repeats make use of +Those with "MIN" in their name are the minimizing versions. Those with "POS" in +their names are possessive versions. Each is followed by the character that is +to be repeated. Other repeats make use of OP_UPTO OP_MINUPTO + OP_POSUPTO OP_EXACT which are followed by a two-byte count (most significant first) and 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). +OP_UPTO (or OP_MINUPTO or OPT_POSUPTO). Repeating character types @@ -147,12 +186,16 @@ byte. 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 @@ -216,9 +259,10 @@ OP_REF is followed by two bytes containing the reference number. Repeating character classes and back references ----------------------------------------------- -Single-character classes are handled specially (see above). This applies to -OP_CLASS and OP_REF. 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 +Single-character classes are handled specially (see above). This section +applies to OP_CLASS and OP_REF. 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 @@ -230,7 +274,9 @@ item. The matching code looks at the following opcode to see if it is one of OP_CRMINRANGE All but the last two are just single-byte items. The others are followed by -four bytes of data, comprising the minimum and maximum repeat counts. +four bytes of data, comprising the minimum and maximum repeat counts. There are +no special possessive opcodes for these repeats; a possessive repeat is +compiled into an atomic group. Brackets and alternation @@ -239,29 +285,25 @@ 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. -Non-capturing brackets use the opcode OP_BRA, while capturing brackets use -OP_BRA+1, OP_BRA+2, etc. [Note for North Americans: "bracket" to some English -speakers, including myself, can be round, square, curly, or pointy. Hence this -usage.] +[Note for North Americans: "bracket" to some English speakers, including +myself, can be round, square, curly, or pointy. Hence this usage.] -Originally PCRE was limited to 99 capturing brackets (so as not to use up all -the opcodes). From release 3.5, there is no limit. What happens is that the -first ones, up to EXTRACT_BASIC_MAX are handled with separate opcodes, as -above. If there are more, the opcode is set to EXTRACT_BASIC_MAX+1, and the -first operation in the bracket is OP_BRANUMBER, followed by a 2-byte bracket -number. This opcode is ignored while matching, but is fished out when handling -the bracket itself. (They could have all been done like this, but I was making -minimal changes.) +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. +the next one, or to the OP_KET opcode. For capturing brackets, the bracket +number immediately follows the offset, always as a 2-byte item. OP_KET is used for subpatterns that do not repeat indefinitely, while OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or maximally respectively. All three are followed by LINK_SIZE bytes giving (as a -positive number) the offset back to the matching OP_BRA opcode. +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 or OP_BRAMINZERO. These are single-byte @@ -276,7 +318,14 @@ 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)?)?)?. +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. Assertions @@ -292,22 +341,27 @@ each alternative of a lookbehind assertion, allowing them to have different fixed lengths. -Once-only subpatterns ---------------------- +Once-only (atomic) subpatterns +------------------------------ These are also just like other subpatterns, but they start with the opcode -OP_ONCE. +OP_ONCE. The check for matching an empty string in an unbounded repeat is +handled entirely at runtime, so there is just this one opcode. Conditional subpatterns ----------------------- -These are like other subpatterns, but they start with the opcode OP_COND. If +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 two bytes containing the -reference number. If the condition is "in recursion" (coded as "(?(R)"), the -same scheme is used, with a "reference number" of 0xffff. Otherwise, a -conditional subpattern always starts with one of the assertions. +reference number. 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, and a value of zero for "the +whole pattern". For a DEFINE condition, just the single byte OP_DEF is used (it +has no associated data). Otherwise, a conditional subpattern always starts with +one of the assertions. Recursion @@ -345,4 +399,4 @@ at compile time, and so does not cause anything to be put into the compiled data. Philip Hazel -June 2006 +November 2006 |