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diff --git a/doc/Tech.Notes b/doc/Tech.Notes index 73c31c7..18eb72b 100644 --- a/doc/Tech.Notes +++ b/doc/Tech.Notes @@ -1,6 +1,9 @@ Technical Notes about PCRE -------------------------- +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 @@ -9,12 +12,15 @@ 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".) When the pattern was all used up, all remaining states were +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. +Historical note 2 +----------------- + By contrast, the code originally written by Henry Spencer and subsequently heavily modified for Perl actually compiles the expression twice: once in a dummy mode in order to find out how much store will be needed, and then for @@ -23,6 +29,9 @@ 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 forms PCRE (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 @@ -38,8 +47,16 @@ got quite big anyway to handle all the Perl stuff. The compiled form of a pattern is a vector of bytes, containing items of variable length. The first byte in an item is an opcode, and the length of the -item is either implicit in the opcode or contained in the data bytes which -follow it. A list of all the opcodes follows: +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 +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. + +A list of all the opcodes follows: Opcodes with no following data ------------------------------ @@ -48,7 +65,7 @@ These items are all just one byte long OP_END end of pattern OP_ANY match any character - OP_ANYBYTE match any single byte, even in UTF-8 mode + OP_ANYBYTE match any single byte, even in UTF-8 mode OP_SOD match start of data: \A OP_SOM, start of match (subject + offset): \G OP_CIRC ^ (start of data, or after \n in multiline) @@ -63,13 +80,14 @@ These items are all just one byte long OP_EODN match end of data or \n at end: \Z OP_EOD match end of data: \z OP_DOLL $ (end of data, or before \n in multiline) - + OP_EXTUNI match an extended Unicode character + Repeating single characters --------------------------- -The common repeats (*, +, ?) when applied to a single character appear as -two-byte items using the following opcodes: +The common repeats (*, +, ?) when applied to a single character use the +following opcodes: OP_STAR OP_MINSTAR @@ -78,6 +96,7 @@ two-byte items using the following opcodes: OP_QUERY OP_MINQUERY +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 @@ -109,39 +128,52 @@ byte. The opcodes are: OP_TYPEEXACT -Matching a character string +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 a single byte that encodes the desired property value. + +Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by two +bytes: OP_PROP or OP_NOTPROP and then the desired property value. + + +Matching literal characters --------------------------- -The OP_CHARS opcode is followed by a one-byte count and then that number of -characters. If there are more than 255 characters in sequence, successive -instances of OP_CHARS are used. +The OP_CHAR opcode is followed by a single character that is to be matched +casefully. For caseless matching, OP_CHARNC is used. In UTF-8 mode, the +character may be more than one byte long. (Earlier versions of PCRE used +multi-character strings, but this was changed to allow some new features to be +added.) Character classes ----------------- -If there is only one character, OP_CHARS is used for a positive class, -and OP_NOT for a negative one (that is, for something like [^a]). However, in -UTF-8 mode, this applies only to characters with values < 128, because OP_NOT -is confined to single bytes. +If there is only one character, OP_CHAR or OP_CHARNC is used for a positive +class, and OP_NOT for a negative one (that is, for something like [^a]). +However, in UTF-8 mode, the use of OP_NOT applies only to characters with +values < 128, because OP_NOT is confined to single bytes. Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated, negated, single-character class. The normal ones (OP_STAR etc.) are used for a repeated positive single-character class. When there's more than one character in a class and all the characters are less -than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a negative +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 bit map containing a 1 bit for every character that is acceptable. The bits are counted from the least significant end of each byte. -The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode, -subject characters with values greater than 256 can be handled correctly. For +The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode, +subject characters with values greater than 256 can be handled correctly. For OP_CLASS they don't match, whereas for OP_NCLASS they do. For classes containing characters with values > 255, OP_XCLASS is used. It optionally uses a bit map (if any characters lie within it), followed by a list -of pairs and single characters. There is a flag character than indicates +of pairs and single characters. There is a flag character than indicates whether it's a positive or a negative class. @@ -192,14 +224,14 @@ the bracket itself. (They could have all been done like this, but I was making minimal changes.) A bracket opcode is followed by two bytes which give the offset to the next -alternative OP_ALT or, if there aren't any branches, to the matching KET +alternative OP_ALT or, if there aren't any branches, to the matching OP_KET opcode. Each OP_ALT is followed by two bytes giving the offset to the next one, -or to the KET opcode. +or to the OP_KET opcode. 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 two bytes giving (as a -positive number) the offset back to the matching BRA opcode. +positive number) the offset back to the matching OP_BRA 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 @@ -207,15 +239,14 @@ opcodes which tell the matcher that skipping this subpattern entirely is a valid branch. A subpattern with an indefinite maximum repetition is replicated in the -compiled data its minimum number of times (or once with a BRAZERO if the -minimum is zero), with the final copy terminating with a KETRMIN or KETRMAX as -appropriate. +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 BRAZERO or BRAMINZERO before -each replication after the minimum, so that, for example, (abc){2,5} is -compiled as (abc)(abc)((abc)((abc)(abc)?)?)?. The 99 and 200 bracket limits do -not apply to these internally generated brackets. +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)?)?)?. Assertions @@ -260,8 +291,11 @@ from the start of the whole pattern. Callout ------- -OP_CALLOUT is followed by one byte of data that holds a callout number in the -range 0 to 255. +OP_CALLOUT is followed by one byte 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 two-byte value giving the offset in the pattern to the +start of the following item, and another two-byte item giving the length of the +next item. Changing options @@ -278,4 +312,4 @@ at compile time, and so does not cause anything to be put into the compiled data. Philip Hazel -August 2003 +September 2004 |