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revision 71 by nigel, Sat Feb 24 21:40:24 2007 UTC revision 87 by nigel, Sat Feb 24 21:41:21 2007 UTC
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1  Technical Notes about PCRE  Technical Notes about PCRE
2  --------------------------  --------------------------
3    
4    Historical note 1
5    -----------------
6    
7  Many years ago I implemented some regular expression functions to an algorithm  Many years ago I implemented some regular expression functions to an algorithm
8  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
9  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 12  form of an expression was known in advan
12  not operate by backtracking, as the original Henry Spencer code and current  not operate by backtracking, as the original Henry Spencer code and current
13  Perl code does, but instead checked all possibilities simultaneously by keeping  Perl code does, but instead checked all possibilities simultaneously by keeping
14  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
15  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
16  algorithm".) When the pattern was all used up, all remaining states were  algorithm". When the pattern was all used up, all remaining states were
17  possible matches, and the one matching the longest subset of the subject string  possible matches, and the one matching the longest subset of the subject string
18  was chosen. This did not necessarily maximize the individual wild portions of  was chosen. This did not necessarily maximize the individual wild portions of
19  the pattern, as is expected in Unix and Perl-style regular expressions.  the pattern, as is expected in Unix and Perl-style regular expressions.
20    
21    Historical note 2
22    -----------------
23    
24  By contrast, the code originally written by Henry Spencer and subsequently  By contrast, the code originally written by Henry Spencer and subsequently
25  heavily modified for Perl actually compiles the expression twice: once in a  heavily modified for Perl actually compiles the expression twice: once in a
26  dummy mode in order to find out how much store will be needed, and then for  dummy mode in order to find out how much store will be needed, and then for
# Line 23  optionally, minimizing in Perl) the amou Line 29  optionally, minimizing in Perl) the amou
29  individual wild portions of the pattern. This is an "NFA algorithm" in Friedl's  individual wild portions of the pattern. This is an "NFA algorithm" in Friedl's
30  terminology.  terminology.
31    
32  For the set of functions that forms PCRE (which are unrelated to those  OK, here's the real stuff
33  mentioned above), I tried at first to invent an algorithm that used an amount  -------------------------
34  of store bounded by a multiple of the number of characters in the pattern, to  
35  save on compiling time. However, because of the greater complexity in Perl  For the set of functions that form the "basic" PCRE library (which are
36  regular expressions, I couldn't do this. In any case, a first pass through the  unrelated to those mentioned above), I tried at first to invent an algorithm
37  pattern is needed, for a number of reasons. PCRE works by running a very  that used an amount of store bounded by a multiple of the number of characters
38  degenerate first pass to calculate a maximum store size, and then a second pass  in the pattern, to save on compiling time. However, because of the greater
39  to do the real compile - which may use a bit less than the predicted amount of  complexity in Perl regular expressions, I couldn't do this. In any case, a
40  store. The idea is that this is going to turn out faster because the first pass  first pass through the pattern is needed, for a number of reasons. PCRE works
41  is degenerate and the second pass can just store stuff straight into the  by running a very degenerate first pass to calculate a maximum store size, and
42  vector. It does make the compiling functions bigger, of course, but they have  then a second pass to do the real compile - which may use a bit less than the
43  got quite big anyway to handle all the Perl stuff.  predicted amount of store. The idea is that this is going to turn out faster
44    because the first pass is degenerate and the second pass can just store stuff
45    straight into the vector, which it knows is big enough. It does make the
46    compiling functions bigger, of course, but they have got quite big anyway to
47    handle all the Perl stuff.
48    
49    Traditional matching function
50    -----------------------------
51    
52    The "traditional", and original, matching function is called pcre_exec(), and
53    it implements an NFA algorithm, similar to the original Henry Spencer algorithm
54    and the way that Perl works. Not surprising, since it is intended to be as
55    compatible with Perl as possible. This is the function most users of PCRE will
56    use most of the time.
57    
58    Supplementary matching function
59    -------------------------------
60    
61    From PCRE 6.0, there is also a supplementary matching function called
62    pcre_dfa_exec(). This implements a DFA matching algorithm that searches
63    simultaneously for all possible matches that start at one point in the subject
64    string. (Going back to my roots: see Historical Note 1 above.) This function
65    intreprets the same compiled pattern data as pcre_exec(); however, not all the
66    facilities are available, and those that are don't always work in quite the
67    same way. See the user documentation for details.
68    
69    Format of compiled patterns
70    ---------------------------
71    
72  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
73  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
74  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
75  follow it. A list of all the opcodes follows:  follow it.
76    
77    In many cases below "two-byte" data values are specified. This is in fact just
78    a default. PCRE can be compiled to use 3-byte or 4-byte values (impairing the
79    performance). This is necessary only when patterns whose compiled length is
80    greater than 64K are going to be processed. In this description, we assume the
81    "normal" compilation options.
82    
83    A list of all the opcodes follows:
84    
85  Opcodes with no following data  Opcodes with no following data
86  ------------------------------  ------------------------------
# Line 48  These items are all just one byte long Line 89  These items are all just one byte long
89    
90    OP_END                 end of pattern    OP_END                 end of pattern
91    OP_ANY                 match any character    OP_ANY                 match any character
92    OP_ANYBYTE             match any single byte, even in UTF-8 mode    OP_ANYBYTE             match any single byte, even in UTF-8 mode
93    OP_SOD                 match start of data: \A    OP_SOD                 match start of data: \A
94    OP_SOM,                start of match (subject + offset): \G    OP_SOM,                start of match (subject + offset): \G
95    OP_CIRC                ^ (start of data, or after \n in multiline)    OP_CIRC                ^ (start of data, or after \n in multiline)
# Line 63  These items are all just one byte long Line 104  These items are all just one byte long
104    OP_EODN                match end of data or \n at end: \Z    OP_EODN                match end of data or \n at end: \Z
105    OP_EOD                 match end of data: \z    OP_EOD                 match end of data: \z
106    OP_DOLL                $ (end of data, or before \n in multiline)    OP_DOLL                $ (end of data, or before \n in multiline)
107      OP_EXTUNI              match an extended Unicode character
108    
109    
110  Repeating single characters  Repeating single characters
111  ---------------------------  ---------------------------
112    
113  The common repeats (*, +, ?) when applied to a single character appear as  The common repeats (*, +, ?) when applied to a single character use the
114  two-byte items using the following opcodes:  following opcodes:
115    
116    OP_STAR    OP_STAR
117    OP_MINSTAR    OP_MINSTAR
# Line 78  two-byte items using the following opcod Line 120  two-byte items using the following opcod
120    OP_QUERY    OP_QUERY
121    OP_MINQUERY    OP_MINQUERY
122    
123    In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.
124  Those with "MIN" in their name are the minimizing versions. Each is followed by  Those with "MIN" in their name are the minimizing versions. Each is followed by
125  the character that is to be repeated. Other repeats make use of  the character that is to be repeated. Other repeats make use of
126    
# Line 109  byte. The opcodes are: Line 152  byte. The opcodes are:
152    OP_TYPEEXACT    OP_TYPEEXACT
153    
154    
155  Matching a character string  Match by Unicode property
156    -------------------------
157    
158    OP_PROP and OP_NOTPROP are used for positive and negative matches of a
159    character by testing its Unicode property (the \p and \P escape sequences).
160    Each is followed by a single byte that encodes the desired property value.
161    
162    Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by two
163    bytes: OP_PROP or OP_NOTPROP and then the desired property value.
164    
165    
166    Matching literal characters
167  ---------------------------  ---------------------------
168    
169  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
170  characters. If there are more than 255 characters in sequence, successive  casefully. For caseless matching, OP_CHARNC is used. In UTF-8 mode, the
171  instances of OP_CHARS are used.  character may be more than one byte long. (Earlier versions of PCRE used
172    multi-character strings, but this was changed to allow some new features to be
173    added.)
174    
175    
176  Character classes  Character classes
177  -----------------  -----------------
178    
179  If there is only one character, OP_CHARS is used for a positive class,  If there is only one character, OP_CHAR or OP_CHARNC is used for a positive
180  and OP_NOT for a negative one (that is, for something like [^a]). However, in  class, and OP_NOT for a negative one (that is, for something like [^a]).
181  UTF-8 mode, this applies only to characters with values < 128, because OP_NOT  However, in UTF-8 mode, the use of OP_NOT applies only to characters with
182  is confined to single bytes.  values < 128, because OP_NOT is confined to single bytes.
183    
184  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,
185  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
186  repeated positive single-character class.  repeated positive single-character class.
187    
188  When there's more than one character in a class and all the characters are less  When there's more than one character in a class and all the characters are less
189  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
190  one. In either case, the opcode is followed by a 32-byte bit map containing a 1  one. In either case, the opcode is followed by a 32-byte bit map containing a 1
191  bit for every character that is acceptable. The bits are counted from the least  bit for every character that is acceptable. The bits are counted from the least
192  significant end of each byte.  significant end of each byte.
193    
194  The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,  The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,
195  subject characters with values greater than 256 can be handled correctly. For  subject characters with values greater than 256 can be handled correctly. For
196  OP_CLASS they don't match, whereas for OP_NCLASS they do.  OP_CLASS they don't match, whereas for OP_NCLASS they do.
197    
198  For classes containing characters with values > 255, OP_XCLASS is used. It  For classes containing characters with values > 255, OP_XCLASS is used. It
199  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
200  of pairs and single characters. There is a flag character than indicates  of pairs and single characters. There is a flag character than indicates
201  whether it's a positive or a negative class.  whether it's a positive or a negative class.
202    
203    
# Line 191  number. This opcode is ignored while mat Line 247  number. This opcode is ignored while mat
247  the bracket itself. (They could have all been done like this, but I was making  the bracket itself. (They could have all been done like this, but I was making
248  minimal changes.)  minimal changes.)
249    
250  A bracket opcode is followed by two bytes which give the offset to the next  A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
251  alternative OP_ALT or, if there aren't any branches, to the matching KET  next alternative OP_ALT or, if there aren't any branches, to the matching
252  opcode. Each OP_ALT is followed by two bytes giving the offset to the next one,  OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
253  or to the KET opcode.  the next one, or to the OP_KET opcode.
254    
255  OP_KET is used for subpatterns that do not repeat indefinitely, while  OP_KET is used for subpatterns that do not repeat indefinitely, while
256  OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or  OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
257  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
258  positive number) the offset back to the matching BRA opcode.  positive number) the offset back to the matching OP_BRA opcode.
259    
260  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
261  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 207  opcodes which tell the matcher that skip Line 263  opcodes which tell the matcher that skip
263  valid branch.  valid branch.
264    
265  A subpattern with an indefinite maximum repetition is replicated in the  A subpattern with an indefinite maximum repetition is replicated in the
266  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
267  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
268  appropriate.  as appropriate.
269    
270  A subpattern with a bounded maximum repetition is replicated in a nested  A subpattern with a bounded maximum repetition is replicated in a nested
271  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
272  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
273  compiled as (abc)(abc)((abc)((abc)(abc)?)?)?. The 99 and 200 bracket limits do  compiled as (abc)(abc)((abc)((abc)(abc)?)?)?.
 not apply to these internally generated brackets.  
274    
275    
276  Assertions  Assertions
# Line 254  Recursion Line 309  Recursion
309    
310  Recursion either matches the current regex, or some subexpression. The opcode  Recursion either matches the current regex, or some subexpression. The opcode
311  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
312  from the start of the whole pattern.  from the start of the whole pattern. From release 6.5, OP_RECURSE is
313    automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
314    broke it). OP_RECURSE is also used for "subroutine" calls, even though they
315    are not strictly a recursion.
316    
317    
318  Callout  Callout
319  -------  -------
320    
321  OP_CALLOUT is followed by one byte of data that holds a callout number in the  OP_CALLOUT is followed by one byte of data that holds a callout number in the
322  range 0 to 255.  range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
323    cases there follows a two-byte value giving the offset in the pattern to the
324    start of the following item, and another two-byte item giving the length of the
325    next item.
326    
327    
328  Changing options  Changing options
# Line 278  at compile time, and so does not cause a Line 339  at compile time, and so does not cause a
339  data.  data.
340    
341  Philip Hazel  Philip Hazel
342  August 2003  January 2006

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