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code/trunk/doc/Tech.Notes revision 63 by nigel, Sat Feb 24 21:40:03 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
8    -----------------
9    
10  Many years ago I implemented some regular expression functions to an algorithm  Many years ago I implemented some regular expression functions to an algorithm
11  suggested by Martin Richards. These were not Unix-like in form, and were quite  suggested by Martin Richards. These were not Unix-like in form, and were quite
12  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
# Line 9  form of an expression was known in advan Line 15  form of an expression was known in advan
15  not operate by backtracking, as the original Henry Spencer code and current  not operate by backtracking, as the original Henry Spencer code and current
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  By contrast, the code originally written by Henry Spencer and subsequently  
25  heavily modified for Perl actually compiles the expression twice: once in a  Historical note 2
26  dummy mode in order to find out how much store will be needed, and then for  -----------------
27  real. The execution function operates by backtracking and maximizing (or,  
28  optionally, minimizing in Perl) the amount of the subject that matches  By contrast, the code originally written by Henry Spencer (which was
29  individual wild portions of the pattern. This is an "NFA algorithm" in Friedl's  subsequently heavily modified for Perl) compiles the expression twice: once in
30  terminology.  a dummy mode in order to find out how much store will be needed, and then for
31    real. (The Perl version probably doesn't do this any more; I'm talking about
32  For the set of functions that forms PCRE (which are unrelated to those  the original library.) The execution function operates by backtracking and
33  mentioned above), I tried at first to invent an algorithm that used an amount  maximizing (or, optionally, minimizing in Perl) the amount of the subject that
34  of store bounded by a multiple of the number of characters in the pattern, to  matches individual wild portions of the pattern. This is an "NFA algorithm" in
35  save on compiling time. However, because of the greater complexity in Perl  Friedl's terminology.
36  regular expressions, I couldn't do this. In any case, a first pass through the  
37  pattern is needed, for a number of reasons. PCRE works by running a very  OK, here's the real stuff
38  degenerate first pass to calculate a maximum store size, and then a second pass  -------------------------
39  to do the real compile - which may use a bit less than the predicted amount of  
40  store. The idea is that this is going to turn out faster because the first pass  For the set of functions that form the "basic" PCRE library (which are
41  is degenerate and the second pass can just store stuff straight into the  unrelated to those mentioned above), I tried at first to invent an algorithm
42  vector. It does make the compiling functions bigger, of course, but they have  that used an amount of store bounded by a multiple of the number of characters
43  got quite big anyway to handle all the Perl stuff.  in the pattern, to save on compiling time. However, because of the greater
44    complexity in Perl regular expressions, I couldn't do this. In any case, a
45    first pass through the pattern is helpful for other reasons.
46    
47    Computing the memory requirement: how it was
48    --------------------------------------------
49    
50    Up to and including release 6.7, PCRE worked by running a very degenerate first
51    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
109  item is either implicit in the opcode or contained in the data bytes which  item is either implicit in the opcode or contained in the data bytes that
110  follow it. A list of all the opcodes follows:  follow it.
111    
112    In many cases below "two-byte" data values are specified. This is in fact just
113    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
116    greater than 64K are going to be processed. In this description, we assume the
117    "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:
121    
122  Opcodes with no following data  Opcodes with no following data
123  ------------------------------  ------------------------------
# Line 48  These items are all just one byte long Line 126  These items are all just one byte long
126    
127    OP_END                 end of pattern    OP_END                 end of pattern
128    OP_ANY                 match any character    OP_ANY                 match any character
129      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
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_RECURSE             match the pattern recursively    OP_EXTUNI              match an extended Unicode character
150      OP_ANYNL               match any Unicode newline sequence
151    
152    
153  Repeating single characters  Repeating single characters
154  ---------------------------  ---------------------------
155    
156  The common repeats (*, +, ?) when applied to a single character appear as  The common repeats (*, +, ?) when applied to a single character use the
157  two-byte items using the following opcodes:  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  Those with "MIN" in their name are the minimizing versions. Each is followed by  In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.
170  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
171    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 99  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    
207  Matching a character string  Match by Unicode property
208    -------------------------
209    
210    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).
212    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
216    three bytes: OP_PROP or OP_NOTPROP and then the desired property type and
217    value.
218    
219    
220    Matching literal characters
221  ---------------------------  ---------------------------
222    
223  The OP_CHARS opcode is followed by a one-byte count and then that number of  The OP_CHAR opcode is followed by a single character that is to be matched
224  characters. If there are more than 255 characters in sequence, successive  casefully. For caseless matching, OP_CHARNC is used. In UTF-8 mode, the
225  instances of OP_CHARS are used.  character may be more than one byte long. (Earlier versions of PCRE used
226    multi-character strings, but this was changed to allow some new features to be
227    added.)
228    
229    
230  Character classes  Character classes
231  -----------------  -----------------
232    
233  When characters less than 256 are involved, OP_CLASS is used for a character  If there is only one character, OP_CHAR or OP_CHARNC is used for a positive
234  class. If there is only one character, OP_CHARS is used for a positive class,  class, and OP_NOT for a negative one (that is, for something like [^a]).
235  and OP_NOT for a negative one (that is, for something like [^a]). However, in  However, in UTF-8 mode, the use of OP_NOT applies only to characters with
236  UTF-8 mode, this applies only to characters with values < 128, because OP_NOT  values < 128, because OP_NOT is confined to single bytes.
 is confined to single bytes.  
237    
238  Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated,  Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated,
239  negated, single-character class. The normal ones (OP_STAR etc.) are used for a  negated, single-character class. The normal ones (OP_STAR etc.) are used for a
240  repeated positive single-character class.  repeated positive single-character class.
241    
242  OP_CLASS is followed by a 32-byte bit map containing a 1 bit for every  When there's more than one character in a class and all the characters are less
243  character that is acceptable. The bits are counted from the least significant  than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a negative
244  end of each byte.  one. In either case, the opcode is followed by a 32-byte bit map containing a 1
245    bit for every character that is acceptable. The bits are counted from the least
246    significant end of each byte.
247    
248    The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,
249    subject characters with values greater than 256 can be handled correctly. For
250    OP_CLASS they don't match, whereas for OP_NCLASS they do.
251    
252  For classes containing characters with values > 255, OP_XCLASS is used. It  For classes containing characters with values > 255, OP_XCLASS is used. It
253  optionally uses a bit map (if any characters lie within it), followed by a list  optionally uses a bit map (if any characters lie within it), followed by a list
254  of pairs and single characters. There is a flag character than indicates  of pairs and single characters. There is a flag character than indicates
255  whether it's a positive or a negative class.  whether it's a positive or a negative class.
256    
257    
# Line 148  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 162  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 171  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 KET  
 opcode. Each OP_ALT is followed by two bytes giving the offset to the next one,  
 or to the 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 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 201  opcodes which tell the matcher that skip Line 316  opcodes which tell the matcher that skip
316  valid branch.  valid branch.
317    
318  A subpattern with an indefinite maximum repetition is replicated in the  A subpattern with an indefinite maximum repetition is replicated in the
319  compiled data its minimum number of times (or once with a BRAZERO if the  compiled data its minimum number of times (or once with OP_BRAZERO if the
320  minimum is zero), with the final copy terminating with a KETRMIN or KETRMAX as  minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
321  appropriate.  as appropriate.
322    
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 BRAZERO or BRAMINZERO before  fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
325  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)?)?)?. The 99 and 200 bracket limits do  compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group
327  not apply to these internally generated brackets.  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 225  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
373    ---------
374    
375    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
377    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
384    -------
385    
386    OP_CALLOUT is followed by one byte of data that holds a callout number in the
387    range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
388    cases there follows a two-byte value giving the offset in the pattern to the
389    start of the following item, and another two-byte item giving the length of the
390    next item.
391    
392    
393  Changing options  Changing options
# Line 257  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  August 2002  June 2007

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