/[pcre]/code/trunk/HACKING

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-revision 49 by nigel,
Sat Feb 24 21:39:33 2007 UTC
+revision 71 by nigel,
Sat Feb 24 21:40:24 2007 UTC
 Line 6 
 suggested by Martin Richards. These were
  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 Henry Spencer and Perl code does, but
+ not operate by backtracking, as the original Henry Spencer code and current
- instead checked all possibilities simultaneously by keeping a list of current
+ Perl code does, but instead checked all possibilities simultaneously by keeping
- states and checking all of them as it advanced through the subject string. (In
+ a list of current states and checking all of them as it advanced through the
- the terminology of Jeffrey Friedl's book, it was a "DFA algorithm".) When the
+ subject string. (In the terminology of Jeffrey Friedl's book, it was a "DFA
- pattern was all used up, all remaining states were possible matches, and the
+ algorithm".) When the pattern was all used up, all remaining states were
- one matching the longest subset of the subject string was chosen. This did not
+ possible matches, and the one matching the longest subset of the subject string
- necessarily maximize the individual wild portions of the pattern, as is
+ was chosen. This did not necessarily maximize the individual wild portions of
- expected in Unix and Perl-style regular expressions.
+ the pattern, as is expected in Unix and Perl-style regular expressions.
  By contrast, the code originally written by Henry Spencer and subsequently
  heavily modified for Perl actually compiles the expression twice: once in a
 Line 28 
 mentioned above), I tried at first to in
  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, in order to find internal flag settings like (?i) at top
+ pattern is needed, for a number of reasons. PCRE works by running a very
- level. So PCRE works by running a very degenerate first pass to calculate a
+ degenerate first pass to calculate a maximum store size, and then a second pass
- maximum store size, and then a second pass to do the real compile - which may
+ to do the real compile - which may use a bit less than the predicted amount of
- use a bit less than the predicted amount of store. The idea is that this is
+ store. The idea is that this is going to turn out faster because the first pass
- going to turn out faster because the first pass is degenerate and the second
+ is degenerate and the second pass can just store stuff straight into the
- pass can just store stuff straight into the vector. It does make the compiling
+ vector. It does make the compiling functions bigger, of course, but they have
- functions bigger, of course, but they have got quite big anyway to handle all
+ got quite big anyway to handle all the Perl stuff.
- 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
-Line 49 
 These items are all just one byte long
+Line 48 
 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_SOD                 match start of data: \A
+   OP_SOM,                start of match (subject + offset): \G
    OP_CIRC                ^ (start of data, or after \n in multiline)
    OP_NOT_WORD_BOUNDARY   \W
    OP_WORD_BOUNDARY       \w
-Line 62 
 These items are all just one byte long
+Line 63 
 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_RECURSE             match the pattern recursively
  Repeating single characters
 Line 120 
 instances of OP_CHARS are used.
  Character classes
  -----------------
- OP_CLASS is used for a character class, provided there are at least two
+ If there is only one character, OP_CHARS is used for a positive class,
- characters in the class. If there is only one character, OP_CHARS is used for a
+ and OP_NOT for a negative one (that is, for something like [^a]). However, in
- positive class, and OP_NOT for a negative one (that is, for something like
+ UTF-8 mode, this applies only to characters with values < 128, because OP_NOT
- [^a]). Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a
+ is confined to single bytes.
- repeated, negated, single-character class. The normal ones (OP_STAR etc.) are
- used for a repeated positive single-character class.
+ 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
- OP_CLASS is followed by a 32-byte bit map containing a 1 bit for every
+ repeated positive single-character class.
- character that is acceptable. The bits are counted from the least significant
- end of each byte.
+ 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
+ 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
+ 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
+ whether it's a positive or a negative class.
  Back references
  ---------------
- OP_REF is followed by a single byte containing the reference number.
+ OP_REF is followed by two bytes containing the reference number.
  Repeating character classes and back references
-Line 163 
 Brackets and alternation
+Line 176 
 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.]
+ 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.)
  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
  opcode. Each OP_ALT is followed by two bytes giving the offset to the next one,
-Line 191 
 appropriate.
+Line 214 
 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 200-bracket limit does not
+ compiled as (abc)(abc)((abc)((abc)(abc)?)?)?. The 99 and 200 bracket limits do
- apply to these internally generated brackets.
+ not apply to these internally generated brackets.
  Assertions
-Line 220 
 Conditional subpatterns
+Line 243 
 Conditional subpatterns
  These are like other subpatterns, but they start with the opcode OP_COND. If
  the condition is a back reference, this is stored at the start of the
- subpattern using the opcode OP_CREF followed by one byte containing the
+ subpattern using the opcode OP_CREF followed by two bytes containing the
- reference number. Otherwise, a conditional subpattern will always start with
+ reference number. If the condition is "in recursion" (coded as "(?(R)"), the
- one of the assertions.
+ same scheme is used, with a "reference number" of 0xffff. 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 an value which is the offset to the starting bracket
+ 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.
  Changing options
  ----------------
- If any of the /i, /m, or /s options are changed within a parenthesized group,
+ If any of the /i, /m, or /s options are changed within a pattern, an OP_OPT
- an OP_OPT opcode is compiled, followed by one byte containing the new settings
+ opcode is compiled, followed by one byte containing the new settings of these
- of these flags. If there are several alternatives in a group, there is an
+ flags. If there are several alternatives, there is an occurrence of OP_OPT at
- occurrence of OP_OPT at the start of all those following the first options
+ the start of all those following the first options change, to set appropriate
- change, to set appropriate options for the start of the alternative.
+ options for the start of the alternative. Immediately after the end of the
- Immediately after the end of the group there is another such item to reset the
+ group there is another such item to reset the flags to their previous values. A
- flags to their previous values. Other changes of flag within the pattern can be
+ change of flag right at the very start of the pattern can be handled entirely
- handled entirely at compile time, and so do not cause anything to be put into
+ at compile time, and so does not cause anything to be put into the compiled
- the compiled data.
+ data.
  Philip Hazel
- August 2000
+ August 2003

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