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revision 53 by nigel, Sat Feb 24 21:39:42 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
10  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
11  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
12  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
13  instead checked all possibilities simultaneously by keeping a list of current  Perl code does, but instead checked all possibilities simultaneously by keeping
14  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
15  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
16  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
17  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
18  necessarily maximize the individual wild portions of the pattern, as is  was chosen. This did not necessarily maximize the individual wild portions of
19  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
# 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, in order to find internal flag settings like (?i) at top  that used an amount of store bounded by a multiple of the number of characters
38  level. So PCRE works by running a very degenerate first pass to calculate a  in the pattern, to save on compiling time. However, because of the greater
39  maximum store size, and then a second pass to do the real compile - which may  complexity in Perl regular expressions, I couldn't do this. In any case, a
40  use a bit less than the predicted amount of store. The idea is that this is  first pass through the pattern is needed, for a number of reasons. PCRE works
41  going to turn out faster because the first pass is degenerate and the second  by running a very degenerate first pass to calculate a maximum store size, and
42  pass can just store stuff straight into the vector. It does make the compiling  then a second pass to do the real compile - which may use a bit less than the
43  functions bigger, of course, but they have got quite big anyway to handle all  predicted amount of store. The idea is that this is going to turn out faster
44  the Perl stuff.  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 49  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
93    OP_SOD                 match start of data: \A    OP_SOD                 match start of data: \A
94      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)
96    OP_NOT_WORD_BOUNDARY   \W    OP_NOT_WORD_BOUNDARY   \W
97    OP_WORD_BOUNDARY       \w    OP_WORD_BOUNDARY       \w
# Line 62  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_RECURSE             match the pattern recursively    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  OP_CLASS is used for a character class, provided there are at least two  If there is only one character, OP_CHAR or OP_CHARNC is used for a positive
180  characters in the class. If there is only one character, OP_CHARS is used for a  class, and OP_NOT for a negative one (that is, for something like [^a]).
181  positive class, and OP_NOT for a negative one (that is, for something like  However, in UTF-8 mode, the use of OP_NOT applies only to characters with
182  [^a]). Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a  values < 128, because OP_NOT is confined to single bytes.
183  repeated, negated, single-character class. The normal ones (OP_STAR etc.) are  
184  used for a repeated positive single-character class.  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
186  OP_CLASS is followed by a 32-byte bit map containing a 1 bit for every  repeated positive single-character class.
187  character that is acceptable. The bits are counted from the least significant  
188  end of each byte.  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
190    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
192    significant end of each byte.
193    
194    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
196    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
199    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
201    whether it's a positive or a negative class.
202    
203    
204  Back references  Back references
# Line 178  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 194  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 231  Conditional subpatterns Line 299  Conditional subpatterns
299  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
300  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
301  subpattern using the opcode OP_CREF followed by two bytes containing the  subpattern using the opcode OP_CREF followed by two bytes containing the
302  reference number. Otherwise, a conditional subpattern will always start with  reference number. If the condition is "in recursion" (coded as "(?(R)"), the
303  one of the assertions.  same scheme is used, with a "reference number" of 0xffff. Otherwise, a
304    conditional subpattern always starts with one of the assertions.
305    
306    
307    Recursion
308    ---------
309    
310    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
312    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
319    -------
320    
321    OP_CALLOUT is followed by one byte of data that holds a callout number in the
322    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
329  ----------------  ----------------
330    
331  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
332  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
333  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
334  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
335  change, to set appropriate options for the start of the alternative.  options for the start of the alternative. Immediately after the end of the
336  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
337  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
338  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
339  the compiled data.  data.
   
340    
341  Philip Hazel  Philip Hazel
342  August 2001  January 2006

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