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revision 460 by ph10, Sun Oct 4 09:27:20 2009 UTC revision 672 by ph10, Tue Aug 23 16:45:55 2011 UTC
# Line 2  Technical Notes about PCRE Line 2  Technical Notes about PCRE
2  --------------------------  --------------------------
3    
4  These are very rough technical notes that record potentially useful information  These are very rough technical notes that record potentially useful information
5  about PCRE internals.  about PCRE internals. For information about testing PCRE, see the pcretest
6    documentation and the comment at the head of the RunTest file.
7    
8    
9  Historical note 1  Historical note 1
10  -----------------  -----------------
# Line 22  the one matching the longest subset of t Line 24  the one matching the longest subset of t
24  not necessarily maximize the individual wild portions of the pattern, as is  not necessarily maximize the individual wild portions of the pattern, as is
25  expected in Unix and Perl-style regular expressions.  expected in Unix and Perl-style regular expressions.
26    
27    
28  Historical note 2  Historical note 2
29  -----------------  -----------------
30    
# Line 34  maximizing (or, optionally, minimizing i Line 37  maximizing (or, optionally, minimizing i
37  matches individual wild portions of the pattern. This is an "NFA algorithm" in  matches individual wild portions of the pattern. This is an "NFA algorithm" in
38  Friedl's terminology.  Friedl's terminology.
39    
40    
41  OK, here's the real stuff  OK, here's the real stuff
42  -------------------------  -------------------------
43    
# Line 44  in the pattern, to save on compiling tim Line 48  in the pattern, to save on compiling tim
48  complexity in Perl regular expressions, I couldn't do this. In any case, a  complexity in Perl regular expressions, I couldn't do this. In any case, a
49  first pass through the pattern is helpful for other reasons.  first pass through the pattern is helpful for other reasons.
50    
51    
52  Computing the memory requirement: how it was  Computing the memory requirement: how it was
53  --------------------------------------------  --------------------------------------------
54    
# Line 54  idea was that this would turn out faster Line 59  idea was that this would turn out faster
59  the first pass is degenerate and the second pass can just store stuff straight  the first pass is degenerate and the second pass can just store stuff straight
60  into the vector, which it knows is big enough.  into the vector, which it knows is big enough.
61    
62    
63  Computing the memory requirement: how it is  Computing the memory requirement: how it is
64  -------------------------------------------  -------------------------------------------
65    
# Line 63  things I did for 6.8 was to fix Yet Anot Line 69  things I did for 6.8 was to fix Yet Anot
69  I had a flash of inspiration as to how I could run the real compile function in  I had a flash of inspiration as to how I could run the real compile function in
70  a "fake" mode that enables it to compute how much memory it would need, while  a "fake" mode that enables it to compute how much memory it would need, while
71  actually only ever using a few hundred bytes of working memory, and without too  actually only ever using a few hundred bytes of working memory, and without too
72  many tests of the mode that might slow it down. So I re-factored the compiling  many tests of the mode that might slow it down. So I refactored the compiling
73  functions to work this way. This got rid of about 600 lines of source. It  functions to work this way. This got rid of about 600 lines of source. It
74  should make future maintenance and development easier. As this was such a major  should make future maintenance and development easier. As this was such a major
75  change, I never released 6.8, instead upping the number to 7.0 (other quite  change, I never released 6.8, instead upping the number to 7.0 (other quite
# Line 75  runs more slowly than before (30% or mor Line 81  runs more slowly than before (30% or mor
81  is doing a full analysis of the pattern. My hope was that this would not be a  is doing a full analysis of the pattern. My hope was that this would not be a
82  big issue, and in the event, nobody has commented on it.  big issue, and in the event, nobody has commented on it.
83    
84    
85  Traditional matching function  Traditional matching function
86  -----------------------------  -----------------------------
87    
# Line 82  The "traditional", and original, matchin Line 89  The "traditional", and original, matchin
89  it implements an NFA algorithm, similar to the original Henry Spencer algorithm  it implements an NFA algorithm, similar to the original Henry Spencer algorithm
90  and the way that Perl works. This is not surprising, since it is intended to be  and the way that Perl works. This is not surprising, since it is intended to be
91  as compatible with Perl as possible. This is the function most users of PCRE  as compatible with Perl as possible. This is the function most users of PCRE
92  will use most of the time.  will use most of the time. From release 8.20, if PCRE is compiled with
93    just-in-time (JIT) support, and studying a compiled pattern with JIT is
94    successful, the JIT code is run instead of the normal pcre_exec() code, but the
95    result is the same.
96    
97    
98  Supplementary matching function  Supplementary matching function
99  -------------------------------  -------------------------------
# Line 101  needed at compile time to produce a trad Line 112  needed at compile time to produce a trad
112  ever active at once. I believe some other regex matchers work this way.  ever active at once. I believe some other regex matchers work this way.
113    
114    
115    Changeable options
116    ------------------
117    
118    The /i, /m, or /s options (PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL) may
119    change in the middle of patterns. From PCRE 8.13, their processing is handled
120    entirely at compile time by generating different opcodes for the different
121    settings. The runtime functions do not need to keep track of an options state
122    any more.
123    
124    
125  Format of compiled patterns  Format of compiled patterns
126  ---------------------------  ---------------------------
127    
# Line 117  greater than 64K are going to be process Line 138  greater than 64K are going to be process
138  "normal" compilation options. Data values that are counts (e.g. for  "normal" compilation options. Data values that are counts (e.g. for
139  quantifiers) are always just two bytes long.  quantifiers) are always just two bytes long.
140    
 A list of the opcodes follows:  
   
   
141  Opcodes with no following data  Opcodes with no following data
142  ------------------------------  ------------------------------
143    
# Line 132  These items are all just one byte long Line 150  These items are all just one byte long
150    OP_SOD                 match start of data: \A    OP_SOD                 match start of data: \A
151    OP_SOM,                start of match (subject + offset): \G    OP_SOM,                start of match (subject + offset): \G
152    OP_SET_SOM,            set start of match (\K)    OP_SET_SOM,            set start of match (\K)
153    OP_CIRC                ^ (start of data, or after \n in multiline)    OP_CIRC                ^ (start of data)
154      OP_CIRCM               ^ multiline mode (start of data or after newline)
155    OP_NOT_WORD_BOUNDARY   \W    OP_NOT_WORD_BOUNDARY   \W
156    OP_WORD_BOUNDARY       \w    OP_WORD_BOUNDARY       \w
157    OP_NOT_DIGIT           \D    OP_NOT_DIGIT           \D
# Line 147  These items are all just one byte long Line 166  These items are all just one byte long
166    OP_WORDCHAR            \w    OP_WORDCHAR            \w
167    OP_EODN                match end of data or \n at end: \Z    OP_EODN                match end of data or \n at end: \Z
168    OP_EOD                 match end of data: \z    OP_EOD                 match end of data: \z
169    OP_DOLL                $ (end of data, or before \n in multiline)    OP_DOLL                $ (end of data, or before final newline)
170      OP_DOLLM               $ multiline mode (end of data or before newline)
171    OP_EXTUNI              match an extended Unicode character    OP_EXTUNI              match an extended Unicode character
172    OP_ANYNL               match any Unicode newline sequence    OP_ANYNL               match any Unicode newline sequence
173    
174    OP_ACCEPT              ) These are Perl 5.10's "backtracking    OP_ACCEPT              ) These are Perl 5.10's "backtracking control
175    OP_COMMIT              ) control verbs". If OP_ACCEPT is inside    OP_COMMIT              ) verbs". If OP_ACCEPT is inside capturing
176    OP_FAIL                ) capturing parentheses, it may be preceded    OP_FAIL                ) parentheses, it may be preceded by one or more
177    OP_PRUNE               ) by one or more OP_CLOSE, followed by a 2-byte    OP_PRUNE               ) OP_CLOSE, followed by a 2-byte number,
178    OP_SKIP                ) number, indicating which parentheses must be    OP_SKIP                ) indicating which parentheses must be closed.
179    OP_THEN                ) closed.  
180    
181    Backtracking control verbs with data
182    ------------------------------------
183    
184    OP_THEN is followed by a LINK_SIZE offset, which is the distance back to the
185    start of the current branch.
186    
187    OP_MARK is followed by the mark name, preceded by a one-byte length, and
188    followed by a binary zero. For (*PRUNE), (*SKIP), and (*THEN) with arguments,
189    the opcodes OP_PRUNE_ARG, OP_SKIP_ARG, and OP_THEN_ARG are used. For the first
190    two, the name follows immediately; for OP_THEN_ARG, it follows the LINK_SIZE
191    offset value.
192    
193    
194    Matching literal characters
195    ---------------------------
196    
197    The OP_CHAR opcode is followed by a single character that is to be matched
198    casefully. For caseless matching, OP_CHARI is used. In UTF-8 mode, the
199    character may be more than one byte long. (Earlier versions of PCRE used
200    multi-character strings, but this was changed to allow some new features to be
201    added.)
202    
203    
204  Repeating single characters  Repeating single characters
205  ---------------------------  ---------------------------
206    
207  The common repeats (*, +, ?) when applied to a single character use the  The common repeats (*, +, ?) when applied to a single character use the
208  following opcodes:  following opcodes, which come in caseful and caseless versions:
209    
210    OP_STAR    Caseful         Caseless
211    OP_MINSTAR    OP_STAR         OP_STARI
212    OP_POSSTAR    OP_MINSTAR      OP_MINSTARI
213    OP_PLUS    OP_POSSTAR      OP_POSSTARI
214    OP_MINPLUS    OP_PLUS         OP_PLUSI
215    OP_POSPLUS    OP_MINPLUS      OP_MINPLUSI
216    OP_QUERY    OP_POSPLUS      OP_POSPLUSI
217    OP_MINQUERY    OP_QUERY        OP_QUERYI
218    OP_POSQUERY    OP_MINQUERY     OP_MINQUERYI
219      OP_POSQUERY     OP_POSQUERYI
220    
221  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.
222  Those with "MIN" in their name are the minimizing versions. Those with "POS" in  Those with "MIN" in their name are the minimizing versions. Those with "POS" in
223  their names are possessive versions. Each is followed by the character that is  their names are possessive versions. Each is followed by the character that is
224  to be repeated. Other repeats make use of  to be repeated. Other repeats make use of these opcodes:
225    
226    OP_UPTO    Caseful         Caseless
227    OP_MINUPTO    OP_UPTO         OP_UPTOI
228    OP_POSUPTO    OP_MINUPTO      OP_MINUPTOI
229    OP_EXACT    OP_POSUPTO      OP_POSUPTOI
230      OP_EXACT        OP_EXACTI
231    
232  which are followed by a two-byte count (most significant first) and the  Each of these is followed by a two-byte count (most significant first) and the
233  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
234  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
235  OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).  OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).
# Line 226  three bytes: OP_PROP or OP_NOTPROP and t Line 270  three bytes: OP_PROP or OP_NOTPROP and t
270  value.  value.
271    
272    
 Matching literal characters  
 ---------------------------  
   
 The OP_CHAR opcode is followed by a single character that is to be matched  
 casefully. For caseless matching, OP_CHARNC is used. In UTF-8 mode, the  
 character may be more than one byte long. (Earlier versions of PCRE used  
 multi-character strings, but this was changed to allow some new features to be  
 added.)  
   
   
273  Character classes  Character classes
274  -----------------  -----------------
275    
276  If there is only one character, OP_CHAR or OP_CHARNC is used for a positive  If there is only one character, OP_CHAR or OP_CHARI is used for a positive
277  class, and OP_NOT for a negative one (that is, for something like [^a]).  class, and OP_NOT or OP_NOTI for a negative one (that is, for something like
278  However, in UTF-8 mode, the use of OP_NOT applies only to characters with  [^a]). However, in UTF-8 mode, the use of OP_NOT[I] applies only to characters
279  values < 128, because OP_NOT is confined to single bytes.  with values < 128, because OP_NOT[I] is confined to single bytes.
280    
281  Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated,  Another set of 13 repeating opcodes (called OP_NOTSTAR etc.) are used for a
282  negated, single-character class. The normal ones (OP_STAR etc.) are used for a  repeated, negated, single-character class. The normal single-character opcodes
283  repeated positive single-character class.  (OP_STAR, etc.) are used for a repeated positive single-character class.
284    
285  When there's more than one character in a class and all the characters are less  When there is more than one character in a class and all the characters are
286  than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a negative  less than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a
287  one. In either case, the opcode is followed by a 32-byte bit map containing a 1  negative one. In either case, the opcode is followed by a 32-byte bit map
288  bit for every character that is acceptable. The bits are counted from the least  containing a 1 bit for every character that is acceptable. The bits are counted
289  significant end of each byte.  from the least significant end of each byte. In caseless mode, bits for both
290    cases are set.
291    
292  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,
293  subject characters with values greater than 256 can be handled correctly. For  subject characters with values greater than 256 can be handled correctly. For
294  OP_CLASS they don't match, whereas for OP_NCLASS they do.  OP_CLASS they do not match, whereas for OP_NCLASS they do.
295    
296  For classes containing characters with values > 255, OP_XCLASS is used. It  For classes containing characters with values > 255, OP_XCLASS is used. It
297  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
298  of pairs and single characters. There is a flag character than indicates  of pairs (for a range) and single characters. In caseless mode, both cases are
299  whether it's a positive or a negative class.  explicitly listed. There is a flag character than indicates whether it is a
300    positive or a negative class.
301    
302    
303  Back references  Back references
304  ---------------  ---------------
305    
306  OP_REF is followed by two bytes containing the reference number.  OP_REF (caseful) or OP_REFI (caseless) is followed by two bytes containing the
307    reference number.
308    
309    
310  Repeating character classes and back references  Repeating character classes and back references
311  -----------------------------------------------  -----------------------------------------------
312    
313  Single-character classes are handled specially (see above). This section  Single-character classes are handled specially (see above). This section
314  applies to OP_CLASS and OP_REF. In both cases, the repeat information follows  applies to OP_CLASS and OP_REF[I]. In both cases, the repeat information
315  the base item. The matching code looks at the following opcode to see if it is  follows the base item. The matching code looks at the following opcode to see
316  one of  if it is one of
317    
318    OP_CRSTAR    OP_CRSTAR
319    OP_CRMINSTAR    OP_CRMINSTAR
# Line 316  number immediately follows the offset, a Line 353  number immediately follows the offset, a
353    
354  OP_KET is used for subpatterns that do not repeat indefinitely, while  OP_KET is used for subpatterns that do not repeat indefinitely, while
355  OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or  OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
356  maximally respectively. All three are followed by LINK_SIZE bytes giving (as a  maximally respectively (see below for possessive repetitions). All three are
357  positive number) the offset back to the matching bracket opcode.  followed by LINK_SIZE bytes giving (as a positive number) the offset back to
358    the matching bracket opcode.
359    
360  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
361  is preceded by one of OP_BRAZERO, OP_BRAMINZERO, or OP_SKIPZERO. These are  is preceded by one of OP_BRAZERO, OP_BRAMINZERO, or OP_SKIPZERO. These are
# Line 344  final replication is changed to OP_SBRA Line 382  final replication is changed to OP_SBRA
382  that it needs to check for matching an empty string when it hits OP_KETRMIN or  that it needs to check for matching an empty string when it hits OP_KETRMIN or
383  OP_KETRMAX, and if so, to break the loop.  OP_KETRMAX, and if so, to break the loop.
384    
385    Possessive brackets
386    -------------------
387    
388    When a repeated group (capturing or non-capturing) is marked as possessive by
389    the "+" notation, e.g. (abc)++, different opcodes are used. Their names all
390    have POS on the end, e.g. OP_BRAPOS instead of OP_BRA and OP_SCPBRPOS instead
391    of OP_SCBRA. The end of such a group is marked by OP_KETRPOS. If the minimum
392    repetition is zero, the group is preceded by OP_BRAPOSZERO.
393    
394    
395  Assertions  Assertions
396  ----------  ----------
# Line 405  start of the following item, and another Line 452  start of the following item, and another
452  next item.  next item.
453    
454    
 Changing options  
 ----------------  
   
 If any of the /i, /m, or /s options are changed within a pattern, an OP_OPT  
 opcode is compiled, followed by one byte containing the new settings of these  
 flags. If there are several alternatives, there is an occurrence of OP_OPT at  
 the start of all those following the first options change, to set appropriate  
 options for the start of the alternative. Immediately after the end of the  
 group there is another such item to reset the flags to their previous values. A  
 change of flag right at the very start of the pattern can be handled entirely  
 at compile time, and so does not cause anything to be put into the compiled  
 data.  
   
455  Philip Hazel  Philip Hazel
456  October 2009  August 2011

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