/[pcre]/code/tags/pcre-5.0/doc/Tech.Notes
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revision 49 by nigel, Sat Feb 24 21:39:33 2007 UTC revision 63 by nigel, Sat Feb 24 21:40:03 2007 UTC
# Line 6  suggested by Martin Richards. These were Line 6  suggested by Martin Richards. These were
6  restricted in what they could do by comparison with Perl. The interesting part  restricted in what they could do by comparison with Perl. The interesting part
7  about the algorithm was that the amount of space required to hold the compiled  about the algorithm was that the amount of space required to hold the compiled
8  form of an expression was known in advance. The code to apply an expression did  form of an expression was known in advance. The code to apply an expression did
9  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
10  instead checked all possibilities simultaneously by keeping a list of current  Perl code does, but instead checked all possibilities simultaneously by keeping
11  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
12  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
13  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
14  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
15  necessarily maximize the individual wild portions of the pattern, as is  was chosen. This did not necessarily maximize the individual wild portions of
16  expected in Unix and Perl-style regular expressions.  the pattern, as is expected in Unix and Perl-style regular expressions.
17    
18  By contrast, the code originally written by Henry Spencer and subsequently  By contrast, the code originally written by Henry Spencer and subsequently
19  heavily modified for Perl actually compiles the expression twice: once in a  heavily modified for Perl actually compiles the expression twice: once in a
# Line 28  mentioned above), I tried at first to in Line 28  mentioned above), I tried at first to in
28  of store bounded by a multiple of the number of characters in the pattern, to  of store bounded by a multiple of the number of characters in the pattern, to
29  save on compiling time. However, because of the greater complexity in Perl  save on compiling time. However, because of the greater complexity in Perl
30  regular expressions, I couldn't do this. In any case, a first pass through the  regular expressions, I couldn't do this. In any case, a first pass through the
31  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
32  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
33  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
34  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
35  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
36  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
37  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.  
38    
39  The compiled form of a pattern is a vector of bytes, containing items of  The compiled form of a pattern is a vector of bytes, containing items of
40  variable length. The first byte in an item is an opcode, and the length of the  variable length. The first byte in an item is an opcode, and the length of the
# Line 120  instances of OP_CHARS are used. Line 119  instances of OP_CHARS are used.
119  Character classes  Character classes
120  -----------------  -----------------
121    
122  OP_CLASS is used for a character class, provided there are at least two  When characters less than 256 are involved, OP_CLASS is used for a character
123  characters in the class. If there is only one character, OP_CHARS is used for a  class. If there is only one character, OP_CHARS is used for a positive class,
124  positive class, and OP_NOT for a negative one (that is, for something like  and OP_NOT for a negative one (that is, for something like [^a]). However, in
125  [^a]). Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a  UTF-8 mode, this applies only to characters with values < 128, because OP_NOT
126  repeated, negated, single-character class. The normal ones (OP_STAR etc.) are  is confined to single bytes.
127  used for a repeated positive single-character class.  
128    Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated,
129    negated, single-character class. The normal ones (OP_STAR etc.) are used for a
130    repeated positive single-character class.
131    
132  OP_CLASS is followed by a 32-byte bit map containing a 1 bit for every  OP_CLASS is followed by a 32-byte bit map containing a 1 bit for every
133  character that is acceptable. The bits are counted from the least significant  character that is acceptable. The bits are counted from the least significant
134  end of each byte.  end of each byte.
135    
136    For classes containing characters with values > 255, OP_XCLASS is used. It
137    optionally uses a bit map (if any characters lie within it), followed by a list
138    of pairs and single characters. There is a flag character than indicates
139    whether it's a positive or a negative class.
140    
141    
142  Back references  Back references
143  ---------------  ---------------
144    
145  OP_REF is followed by a single byte containing the reference number.  OP_REF is followed by two bytes containing the reference number.
146    
147    
148  Repeating character classes and back references  Repeating character classes and back references
# Line 163  Brackets and alternation Line 170  Brackets and alternation
170    
171  A pair of non-capturing (round) brackets is wrapped round each expression at  A pair of non-capturing (round) brackets is wrapped round each expression at
172  compile time, so alternation always happens in the context of brackets.  compile time, so alternation always happens in the context of brackets.
173    
174  Non-capturing brackets use the opcode OP_BRA, while capturing brackets use  Non-capturing brackets use the opcode OP_BRA, while capturing brackets use
175  OP_BRA+1, OP_BRA+2, etc. [Note for North Americans: "bracket" to some English  OP_BRA+1, OP_BRA+2, etc. [Note for North Americans: "bracket" to some English
176  speakers, including myself, can be round, square, curly, or pointy. Hence this  speakers, including myself, can be round, square, curly, or pointy. Hence this
177  usage.]  usage.]
178    
179    Originally PCRE was limited to 99 capturing brackets (so as not to use up all
180    the opcodes). From release 3.5, there is no limit. What happens is that the
181    first ones, up to EXTRACT_BASIC_MAX are handled with separate opcodes, as
182    above. If there are more, the opcode is set to EXTRACT_BASIC_MAX+1, and the
183    first operation in the bracket is OP_BRANUMBER, followed by a 2-byte bracket
184    number. This opcode is ignored while matching, but is fished out when handling
185    the bracket itself. (They could have all been done like this, but I was making
186    minimal changes.)
187    
188  A bracket opcode is followed by two bytes which give the offset to the next  A bracket opcode is followed by two bytes which give the offset to the next
189  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 KET
190  opcode. Each OP_ALT is followed by two bytes giving the offset to the next one,  opcode. Each OP_ALT is followed by two bytes giving the offset to the next one,
# Line 191  appropriate. Line 208  appropriate.
208  A subpattern with a bounded maximum repetition is replicated in a nested  A subpattern with a bounded maximum repetition is replicated in a nested
209  fashion up to the maximum number of times, with BRAZERO or BRAMINZERO before  fashion up to the maximum number of times, with BRAZERO or BRAMINZERO before
210  each replication after the minimum, so that, for example, (abc){2,5} is  each replication after the minimum, so that, for example, (abc){2,5} is
211  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
212  apply to these internally generated brackets.  not apply to these internally generated brackets.
213    
214    
215  Assertions  Assertions
# Line 220  Conditional subpatterns Line 237  Conditional subpatterns
237    
238  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. If
239  the condition is a back reference, this is stored at the start of the  the condition is a back reference, this is stored at the start of the
240  subpattern using the opcode OP_CREF followed by one byte containing the  subpattern using the opcode OP_CREF followed by two bytes containing the
241  reference number. Otherwise, a conditional subpattern will always start with  reference number. If the condition is "in recursion" (coded as "(?(R)"), the
242  one of the assertions.  same scheme is used, with a "reference number" of 0xffff. Otherwise, a
243    conditional subpattern always starts with one of the assertions.
244    
245    
246  Changing options  Changing options
247  ----------------  ----------------
248    
249  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
250  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
251  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
252  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
253  change, to set appropriate options for the start of the alternative.  options for the start of the alternative. Immediately after the end of the
254  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
255  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
256  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
257  the compiled data.  data.
   
258    
259  Philip Hazel  Philip Hazel
260  August 2000  August 2002
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