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code/trunk/doc/Tech.Notes revision 75 by nigel, Sat Feb 24 21:40:37 2007 UTC code/trunk/HACKING revision 181 by ph10, Wed Jun 13 14:55:18 2007 UTC
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1  Technical Notes about PCRE  Technical Notes about PCRE
2  --------------------------  --------------------------
3    
4    These are very rough technical notes that record potentially useful information
5    about PCRE internals.
6    
7  Historical note 1  Historical note 1
8  -----------------  -----------------
9    
# Line 13  not operate by backtracking, as the orig Line 16  not operate by backtracking, as the orig
16  Perl code does, but instead checked all possibilities simultaneously by keeping  Perl code does, but instead checked all possibilities simultaneously by keeping
17  a list of current states and checking all of them as it advanced through the  a list of current states and checking all of them as it advanced through the
18  subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA  subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA
19  algorithm". When the pattern was all used up, all remaining states were  algorithm", though it was not a traditional Finite State Machine (FSM). When
20  possible matches, and the one matching the longest subset of the subject string  the pattern was all used up, all remaining states were possible matches, and
21  was chosen. This did not necessarily maximize the individual wild portions of  the one matching the longest subset of the subject string was chosen. This did
22  the pattern, as is expected in Unix and Perl-style regular expressions.  not necessarily maximize the individual wild portions of the pattern, as is
23    expected in Unix and Perl-style regular expressions.
24    
25  Historical note 2  Historical note 2
26  -----------------  -----------------
27    
28  By contrast, the code originally written by Henry Spencer and subsequently  By contrast, the code originally written by Henry Spencer (which was
29  heavily modified for Perl actually compiles the expression twice: once in a  subsequently heavily modified for Perl) compiles the expression twice: once in
30  dummy mode in order to find out how much store will be needed, and then for  a dummy mode in order to find out how much store will be needed, and then for
31  real. The execution function operates by backtracking and maximizing (or,  real. (The Perl version probably doesn't do this any more; I'm talking about
32  optionally, minimizing in Perl) the amount of the subject that matches  the original library.) The execution function operates by backtracking and
33  individual wild portions of the pattern. This is an "NFA algorithm" in Friedl's  maximizing (or, optionally, minimizing in Perl) the amount of the subject that
34  terminology.  matches individual wild portions of the pattern. This is an "NFA algorithm" in
35    Friedl's terminology.
36    
37  OK, here's the real stuff  OK, here's the real stuff
38  -------------------------  -------------------------
39    
40  For the set of functions that forms PCRE (which are unrelated to those  For the set of functions that form the "basic" PCRE library (which are
41  mentioned above), I tried at first to invent an algorithm that used an amount  unrelated to those mentioned above), I tried at first to invent an algorithm
42  of store bounded by a multiple of the number of characters in the pattern, to  that used an amount of store bounded by a multiple of the number of characters
43  save on compiling time. However, because of the greater complexity in Perl  in the pattern, to save on compiling time. However, because of the greater
44  regular expressions, I couldn't do this. In any case, a first pass through the  complexity in Perl regular expressions, I couldn't do this. In any case, a
45  pattern is needed, for a number of reasons. PCRE works by running a very  first pass through the pattern is helpful for other reasons.
46  degenerate first pass to calculate a maximum store size, and then a second pass  
47  to do the real compile - which may use a bit less than the predicted amount of  Computing the memory requirement: how it was
48  store. The idea is that this is going to turn out faster because the first pass  --------------------------------------------
49  is degenerate and the second pass can just store stuff straight into the  
50  vector. It does make the compiling functions bigger, of course, but they have  Up to and including release 6.7, PCRE worked by running a very degenerate first
51  got quite big anyway to handle all the Perl stuff.  pass to calculate a maximum store size, and then a second pass to do the real
52    compile - which might use a bit less than the predicted amount of memory. The
53    idea was that this would turn out faster than the Henry Spencer code because
54    the first pass is degenerate and the second pass can just store stuff straight
55    into the vector, which it knows is big enough.
56    
57    Computing the memory requirement: how it is
58    -------------------------------------------
59    
60    By the time I was working on a potential 6.8 release, the degenerate first pass
61    had become very complicated and hard to maintain. Indeed one of the early
62    things I did for 6.8 was to fix Yet Another Bug in the memory computation. Then
63    I had a flash of inspiration as to how I could run the real compile function in
64    a "fake" mode that enables it to compute how much memory it would need, while
65    actually only ever using a few hundred bytes of working memory, and without too
66    many tests of the mode that might slow it down. So I re-factored the compiling
67    functions to work this way. This got rid of about 600 lines of source. It
68    should make future maintenance and development easier. As this was such a major
69    change, I never released 6.8, instead upping the number to 7.0 (other quite
70    major changes are also present in the 7.0 release).
71    
72    A side effect of this work is that the previous limit of 200 on the nesting
73    depth of parentheses was removed. However, there is a downside: pcre_compile()
74    runs more slowly than before (30% or more, depending on the pattern) because it
75    is doing a full analysis of the pattern. My hope is that this is not a big
76    issue.
77    
78    Traditional matching function
79    -----------------------------
80    
81    The "traditional", and original, matching function is called pcre_exec(), and
82    it implements an NFA algorithm, similar to the original Henry Spencer algorithm
83    and the way that Perl works. Not surprising, since it is intended to be as
84    compatible with Perl as possible. This is the function most users of PCRE will
85    use most of the time.
86    
87    Supplementary matching function
88    -------------------------------
89    
90    From PCRE 6.0, there is also a supplementary matching function called
91    pcre_dfa_exec(). This implements a DFA matching algorithm that searches
92    simultaneously for all possible matches that start at one point in the subject
93    string. (Going back to my roots: see Historical Note 1 above.) This function
94    intreprets the same compiled pattern data as pcre_exec(); however, not all the
95    facilities are available, and those that are do not always work in quite the
96    same way. See the user documentation for details.
97    
98    The algorithm that is used for pcre_dfa_exec() is not a traditional FSM,
99    because it may have a number of states active at one time. More work would be
100    needed at compile time to produce a traditional FSM where only one state is
101    ever active at once. I believe some other regex matchers work this way.
102    
103    
104    Format of compiled patterns
105    ---------------------------
106    
107  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
108  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 51  item is either implicit in the opcode or Line 110  item is either implicit in the opcode or
110  follow it.  follow it.
111    
112  In many cases below "two-byte" data values are specified. This is in fact just  In many cases below "two-byte" data values are specified. This is in fact just
113  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
114    compiled to use 3-byte or 4-byte values for these offsets (impairing the
115  performance). This is necessary only when patterns whose compiled length is  performance). This is necessary only when patterns whose compiled length is
116  greater than 64K are going to be processed. In this description, we assume the  greater than 64K are going to be processed. In this description, we assume the
117  "normal" compilation options.  "normal" compilation options. "Two-byte" data values that are counts (e.g. for
118    quantifiers) are always just two bytes.
119    
120  A list of all the opcodes follows:  A list of all the opcodes follows:
121    
# Line 68  These items are all just one byte long Line 129  These items are all just one byte long
129    OP_ANYBYTE             match any single byte, even in UTF-8 mode    OP_ANYBYTE             match any single byte, even in UTF-8 mode
130    OP_SOD                 match start of data: \A    OP_SOD                 match start of data: \A
131    OP_SOM,                start of match (subject + offset): \G    OP_SOM,                start of match (subject + offset): \G
132      OP_SET_SOM,            set start of match (\K)
133    OP_CIRC                ^ (start of data, or after \n in multiline)    OP_CIRC                ^ (start of data, or after \n in multiline)
134    OP_NOT_WORD_BOUNDARY   \W    OP_NOT_WORD_BOUNDARY   \W
135    OP_WORD_BOUNDARY       \w    OP_WORD_BOUNDARY       \w
136    OP_NOT_DIGIT           \D    OP_NOT_DIGIT           \D
137    OP_DIGIT               \d    OP_DIGIT               \d
138      OP_NOT_HSPACE          \H
139      OP_HSPACE              \h
140    OP_NOT_WHITESPACE      \S    OP_NOT_WHITESPACE      \S
141    OP_WHITESPACE          \s    OP_WHITESPACE          \s
142      OP_NOT_VSPACE          \V
143      OP_VSPACE              \v
144    OP_NOT_WORDCHAR        \W    OP_NOT_WORDCHAR        \W
145    OP_WORDCHAR            \w    OP_WORDCHAR            \w
146    OP_EODN                match end of data or \n at end: \Z    OP_EODN                match end of data or \n at end: \Z
147    OP_EOD                 match end of data: \z    OP_EOD                 match end of data: \z
148    OP_DOLL                $ (end of data, or before \n in multiline)    OP_DOLL                $ (end of data, or before \n in multiline)
149    OP_EXTUNI              match an extended Unicode character    OP_EXTUNI              match an extended Unicode character
150      OP_ANYNL               match any Unicode newline sequence
151    
152    
153  Repeating single characters  Repeating single characters
# Line 91  following opcodes: Line 158  following opcodes:
158    
159    OP_STAR    OP_STAR
160    OP_MINSTAR    OP_MINSTAR
161      OP_POSSTAR
162    OP_PLUS    OP_PLUS
163    OP_MINPLUS    OP_MINPLUS
164      OP_POSPLUS
165    OP_QUERY    OP_QUERY
166    OP_MINQUERY    OP_MINQUERY
167      OP_POSQUERY
168    
169  In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.  In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.
170  Those with "MIN" in their name are the minimizing versions. Each is followed by  Those with "MIN" in their name are the minimizing versions. Those with "POS" in
171  the character that is to be repeated. Other repeats make use of  their names are possessive versions. Each is followed by the character that is
172    to be repeated. Other repeats make use of
173    
174    OP_UPTO    OP_UPTO
175    OP_MINUPTO    OP_MINUPTO
176      OP_POSUPTO
177    OP_EXACT    OP_EXACT
178    
179  which are followed by a two-byte count (most significant first) and the  which are followed by a two-byte count (most significant first) and the
180  repeated character. OP_UPTO matches from 0 to the given number. A repeat with a  repeated character. OP_UPTO matches from 0 to the given number. A repeat with a
181  non-zero minimum and a fixed maximum is coded as an OP_EXACT followed by an  non-zero minimum and a fixed maximum is coded as an OP_EXACT followed by an
182  OP_UPTO (or OP_MINUPTO).  OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).
183    
184    
185  Repeating character types  Repeating character types
# Line 119  byte. The opcodes are: Line 191  byte. The opcodes are:
191    
192    OP_TYPESTAR    OP_TYPESTAR
193    OP_TYPEMINSTAR    OP_TYPEMINSTAR
194      OP_TYPEPOSSTAR
195    OP_TYPEPLUS    OP_TYPEPLUS
196    OP_TYPEMINPLUS    OP_TYPEMINPLUS
197      OP_TYPEPOSPLUS
198    OP_TYPEQUERY    OP_TYPEQUERY
199    OP_TYPEMINQUERY    OP_TYPEMINQUERY
200      OP_TYPEPOSQUERY
201    OP_TYPEUPTO    OP_TYPEUPTO
202    OP_TYPEMINUPTO    OP_TYPEMINUPTO
203      OP_TYPEPOSUPTO
204    OP_TYPEEXACT    OP_TYPEEXACT
205    
206    
# Line 133  Match by Unicode property Line 209  Match by Unicode property
209    
210  OP_PROP and OP_NOTPROP are used for positive and negative matches of a  OP_PROP and OP_NOTPROP are used for positive and negative matches of a
211  character by testing its Unicode property (the \p and \P escape sequences).  character by testing its Unicode property (the \p and \P escape sequences).
212  Each is followed by a single byte that encodes the desired property value.  Each is followed by two bytes that encode the desired property as a type and a
213    value.
214    
215  Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by two  Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by
216  bytes: OP_PROP or OP_NOTPROP and then the desired property value.  three bytes: OP_PROP or OP_NOTPROP and then the desired property type and
217    value.
218    
219    
220  Matching literal characters  Matching literal characters
# Line 186  OP_REF is followed by two bytes containi Line 264  OP_REF is followed by two bytes containi
264  Repeating character classes and back references  Repeating character classes and back references
265  -----------------------------------------------  -----------------------------------------------
266    
267  Single-character classes are handled specially (see above). This applies to  Single-character classes are handled specially (see above). This section
268  OP_CLASS and OP_REF. In both cases, the repeat information follows the base  applies to OP_CLASS and OP_REF. In both cases, the repeat information follows
269  item. The matching code looks at the following opcode to see if it is one of  the base item. The matching code looks at the following opcode to see if it is
270    one of
271    
272    OP_CRSTAR    OP_CRSTAR
273    OP_CRMINSTAR    OP_CRMINSTAR
# Line 200  item. The matching code looks at the fol Line 279  item. The matching code looks at the fol
279    OP_CRMINRANGE    OP_CRMINRANGE
280    
281  All but the last two are just single-byte items. The others are followed by  All but the last two are just single-byte items. The others are followed by
282  four bytes of data, comprising the minimum and maximum repeat counts.  four bytes of data, comprising the minimum and maximum repeat counts. There are
283    no special possessive opcodes for these repeats; a possessive repeat is
284    compiled into an atomic group.
285    
286    
287  Brackets and alternation  Brackets and alternation
# Line 209  Brackets and alternation Line 290  Brackets and alternation
290  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
291  compile time, so alternation always happens in the context of brackets.  compile time, so alternation always happens in the context of brackets.
292    
293  Non-capturing brackets use the opcode OP_BRA, while capturing brackets use  [Note for North Americans: "bracket" to some English speakers, including
294  OP_BRA+1, OP_BRA+2, etc. [Note for North Americans: "bracket" to some English  myself, can be round, square, curly, or pointy. Hence this usage.]
295  speakers, including myself, can be round, square, curly, or pointy. Hence this  
296  usage.]  Non-capturing brackets use the opcode OP_BRA. Originally PCRE was limited to 99
297    capturing brackets and it used a different opcode for each one. From release
298  Originally PCRE was limited to 99 capturing brackets (so as not to use up all  3.5, the limit was removed by putting the bracket number into the data for
299  the opcodes). From release 3.5, there is no limit. What happens is that the  higher-numbered brackets. From release 7.0 all capturing brackets are handled
300  first ones, up to EXTRACT_BASIC_MAX are handled with separate opcodes, as  this way, using the single opcode OP_CBRA.
301  above. If there are more, the opcode is set to EXTRACT_BASIC_MAX+1, and the  
302  first operation in the bracket is OP_BRANUMBER, followed by a 2-byte bracket  A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
303  number. This opcode is ignored while matching, but is fished out when handling  next alternative OP_ALT or, if there aren't any branches, to the matching
304  the bracket itself. (They could have all been done like this, but I was making  OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
305  minimal changes.)  the next one, or to the OP_KET opcode. For capturing brackets, the bracket
306    number immediately follows the offset, always as a 2-byte item.
 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 OP_KET  
 opcode. Each OP_ALT is followed by two bytes giving the offset to the next one,  
 or to the OP_KET opcode.  
307    
308  OP_KET is used for subpatterns that do not repeat indefinitely, while  OP_KET is used for subpatterns that do not repeat indefinitely, while
309  OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or  OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
310  maximally respectively. All three are followed by two bytes giving (as a  maximally respectively. All three are followed by LINK_SIZE bytes giving (as a
311  positive number) the offset back to the matching OP_BRA opcode.  positive number) the offset back to the matching bracket opcode.
312    
313  If a subpattern is quantified such that it is permitted to match zero times, it  If a subpattern is quantified such that it is permitted to match zero times, it
314  is preceded by one of OP_BRAZERO or OP_BRAMINZERO. These are single-byte  is preceded by one of OP_BRAZERO or OP_BRAMINZERO. These are single-byte
# Line 246  as appropriate. Line 323  as appropriate.
323  A subpattern with a bounded maximum repetition is replicated in a nested  A subpattern with a bounded maximum repetition is replicated in a nested
324  fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO  fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
325  before each replication after the minimum, so that, for example, (abc){2,5} is  before each replication after the minimum, so that, for example, (abc){2,5} is
326  compiled as (abc)(abc)((abc)((abc)(abc)?)?)?.  compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group
327    has the same number.
328    
329    When a repeated subpattern has an unbounded upper limit, it is checked to see
330    whether it could match an empty string. If this is the case, the opcode in the
331    final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher
332    that it needs to check for matching an empty string when it hits OP_KETRMIN or
333    OP_KETRMAX, and if so, to break the loop.
334    
335    
336  Assertions  Assertions
# Line 262  each alternative of a lookbehind asserti Line 346  each alternative of a lookbehind asserti
346  fixed lengths.  fixed lengths.
347    
348    
349  Once-only subpatterns  Once-only (atomic) subpatterns
350  ---------------------  ------------------------------
351    
352  These are also just like other subpatterns, but they start with the opcode  These are also just like other subpatterns, but they start with the opcode
353  OP_ONCE.  OP_ONCE. The check for matching an empty string in an unbounded repeat is
354    handled entirely at runtime, so there is just this one opcode.
355    
356    
357  Conditional subpatterns  Conditional subpatterns
358  -----------------------  -----------------------
359    
360  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
361    OP_SCOND for one that might match an empty string in an unbounded repeat. If
362  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
363  subpattern using the opcode OP_CREF followed by two bytes containing the  subpattern using the opcode OP_CREF followed by two bytes containing the
364  reference number. If the condition is "in recursion" (coded as "(?(R)"), the  reference number. If the condition is "in recursion" (coded as "(?(R)"), or "in
365  same scheme is used, with a "reference number" of 0xffff. Otherwise, a  recursion of group x" (coded as "(?(Rx)"), the group number is stored at the
366  conditional subpattern always starts with one of the assertions.  start of the subpattern using the opcode OP_RREF, and a value of zero for "the
367    whole pattern". For a DEFINE condition, just the single byte OP_DEF is used (it
368    has no associated data). Otherwise, a conditional subpattern always starts with
369    one of the assertions.
370    
371    
372  Recursion  Recursion
# Line 285  Recursion Line 374  Recursion
374    
375  Recursion either matches the current regex, or some subexpression. The opcode  Recursion either matches the current regex, or some subexpression. The opcode
376  OP_RECURSE is followed by an value which is the offset to the starting bracket  OP_RECURSE is followed by an value which is the offset to the starting bracket
377  from the start of the whole pattern.  from the start of the whole pattern. From release 6.5, OP_RECURSE is
378    automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
379    broke it). OP_RECURSE is also used for "subroutine" calls, even though they
380    are not strictly a recursion.
381    
382    
383  Callout  Callout
# Line 312  at compile time, and so does not cause a Line 404  at compile time, and so does not cause a
404  data.  data.
405    
406  Philip Hazel  Philip Hazel
407  September 2004  June 2007

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