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code/trunk/HACKING revision 212 by ph10, Thu Aug 9 11:16:34 2007 UTC code/branches/pcre16/HACKING revision 755 by ph10, Mon Nov 21 10:41:54 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
76  major changes are also present in the 7.0 release).  major changes were also present in the 7.0 release).
77    
78  A side effect of this work is that the previous limit of 200 on the nesting  A side effect of this work was that the previous limit of 200 on the nesting
79  depth of parentheses was removed. However, there is a downside: pcre_compile()  depth of parentheses was removed. However, there is a downside: pcre_compile()
80  runs more slowly than before (30% or more, depending on the pattern) because it  runs more slowly than before (30% or more, depending on the pattern) because it
81  is doing a full analysis of the pattern. My hope is that this is not a big  is doing a full analysis of the pattern. My hope was that this would not be a
82  issue.  big issue, and in the event, nobody has commented on it.
83    
84    
85  Traditional matching function  Traditional matching function
86  -----------------------------  -----------------------------
87    
88  The "traditional", and original, matching function is called pcre_exec(), and  The "traditional", and original, matching function is called pcre_exec(), and
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. Not surprising, since it is intended to be as  and the way that Perl works. This is not surprising, since it is intended to be
91  compatible with Perl as possible. This is the function most users of PCRE will  as compatible with Perl as possible. This is the function most users of PCRE
92  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    
144  These items are all just one byte long  These items are all just one byte long
145    
146    OP_END                 end of pattern    OP_END                 end of pattern
147    OP_ANY                 match any character    OP_ANY                 match any one character other than newline
148      OP_ALLANY              match any one character, including newline
149    OP_ANYBYTE             match any single byte, even in UTF-8 mode    OP_ANYBYTE             match any single byte, even in UTF-8 mode
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 145  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              )    OP_ACCEPT              ) These are Perl 5.10's "backtracking control
175    OP_COMMIT              )    OP_COMMIT              ) verbs". If OP_ACCEPT is inside capturing
176    OP_FAIL                ) These are Perl 5.10's "backtracking    OP_FAIL                ) parentheses, it may be preceded by one or more
177    OP_PRUNE               ) control verbs".    OP_PRUNE               ) OP_CLOSE, followed by a 2-byte number,
178    OP_SKIP                )    OP_SKIP                ) indicating which parentheses must be closed.
179    OP_THEN                )  
180    
181    Backtracking control verbs with (optional) data
182    -----------------------------------------------
183    
184    (*THEN) without an argument generates the opcode OP_THEN and no following data.
185    OP_MARK is followed by the mark name, preceded by a one-byte length, and
186    followed by a binary zero. For (*PRUNE), (*SKIP), and (*THEN) with arguments,
187    the opcodes OP_PRUNE_ARG, OP_SKIP_ARG, and OP_THEN_ARG are used, with the name
188    following in the same format.
189    
190    
191    Matching literal characters
192    ---------------------------
193    
194    The OP_CHAR opcode is followed by a single character that is to be matched
195    casefully. For caseless matching, OP_CHARI is used. In UTF-8 mode, the
196    character may be more than one byte long. (Earlier versions of PCRE used
197    multi-character strings, but this was changed to allow some new features to be
198    added.)
199    
200    
201  Repeating single characters  Repeating single characters
202  ---------------------------  ---------------------------
203    
204  The common repeats (*, +, ?) when applied to a single character use the  The common repeats (*, +, ?) when applied to a single character use the
205  following opcodes:  following opcodes, which come in caseful and caseless versions:
206    
207    OP_STAR    Caseful         Caseless
208    OP_MINSTAR    OP_STAR         OP_STARI
209    OP_POSSTAR    OP_MINSTAR      OP_MINSTARI
210    OP_PLUS    OP_POSSTAR      OP_POSSTARI
211    OP_MINPLUS    OP_PLUS         OP_PLUSI
212    OP_POSPLUS    OP_MINPLUS      OP_MINPLUSI
213    OP_QUERY    OP_POSPLUS      OP_POSPLUSI
214    OP_MINQUERY    OP_QUERY        OP_QUERYI
215    OP_POSQUERY    OP_MINQUERY     OP_MINQUERYI
216      OP_POSQUERY     OP_POSQUERYI
217    
218  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.
219  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
220  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
221  to be repeated. Other repeats make use of  to be repeated. Other repeats make use of these opcodes:
222    
223    OP_UPTO    Caseful         Caseless
224    OP_MINUPTO    OP_UPTO         OP_UPTOI
225    OP_POSUPTO    OP_MINUPTO      OP_MINUPTOI
226    OP_EXACT    OP_POSUPTO      OP_POSUPTOI
227      OP_EXACT        OP_EXACTI
228    
229  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
230  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
231  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
232  OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).  OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).
# Line 224  three bytes: OP_PROP or OP_NOTPROP and t Line 267  three bytes: OP_PROP or OP_NOTPROP and t
267  value.  value.
268    
269    
 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.)  
   
   
270  Character classes  Character classes
271  -----------------  -----------------
272    
273  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
274  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
275  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
276  values < 128, because OP_NOT is confined to single bytes.  with values < 128, because OP_NOT[I] is confined to single bytes.
277    
278  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
279  negated, single-character class. The normal ones (OP_STAR etc.) are used for a  repeated, negated, single-character class. The normal single-character opcodes
280  repeated positive single-character class.  (OP_STAR, etc.) are used for a repeated positive single-character class.
281    
282  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
283  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
284  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
285  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
286  significant end of each byte.  from the least significant end of each byte. In caseless mode, bits for both
287    cases are set.
288    
289  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,
290  subject characters with values greater than 256 can be handled correctly. For  subject characters with values greater than 256 can be handled correctly. For
291  OP_CLASS they don't match, whereas for OP_NCLASS they do.  OP_CLASS they do not match, whereas for OP_NCLASS they do.
292    
293  For classes containing characters with values > 255, OP_XCLASS is used. It  For classes containing characters with values > 255, OP_XCLASS is used. It
294  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
295  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
296  whether it's a positive or a negative class.  explicitly listed. There is a flag character than indicates whether it is a
297    positive or a negative class.
298    
299    
300  Back references  Back references
301  ---------------  ---------------
302    
303  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
304    reference number.
305    
306    
307  Repeating character classes and back references  Repeating character classes and back references
308  -----------------------------------------------  -----------------------------------------------
309    
310  Single-character classes are handled specially (see above). This section  Single-character classes are handled specially (see above). This section
311  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
312  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
313  one of  if it is one of
314    
315    OP_CRSTAR    OP_CRSTAR
316    OP_CRMINSTAR    OP_CRMINSTAR
# Line 314  number immediately follows the offset, a Line 350  number immediately follows the offset, a
350    
351  OP_KET is used for subpatterns that do not repeat indefinitely, while  OP_KET is used for subpatterns that do not repeat indefinitely, while
352  OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or  OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
353  maximally respectively. All three are followed by LINK_SIZE bytes giving (as a  maximally respectively (see below for possessive repetitions). All three are
354  positive number) the offset back to the matching bracket opcode.  followed by LINK_SIZE bytes giving (as a positive number) the offset back to
355    the matching bracket opcode.
356    
357  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
358  is preceded by one of OP_BRAZERO or OP_BRAMINZERO. These are single-byte  is preceded by one of OP_BRAZERO, OP_BRAMINZERO, or OP_SKIPZERO. These are
359  opcodes which tell the matcher that skipping this subpattern entirely is a  single-byte opcodes that tell the matcher that skipping the following
360  valid branch.  subpattern entirely is a valid branch. In the case of the first two, not
361    skipping the pattern is also valid (greedy and non-greedy). The third is used
362    when a pattern has the quantifier {0,0}. It cannot be entirely discarded,
363    because it may be called as a subroutine from elsewhere in the regex.
364    
365  A subpattern with an indefinite maximum repetition is replicated in the  A subpattern with an indefinite maximum repetition is replicated in the
366  compiled data its minimum number of times (or once with OP_BRAZERO if the  compiled data its minimum number of times (or once with OP_BRAZERO if the
# Line 339  final replication is changed to OP_SBRA Line 379  final replication is changed to OP_SBRA
379  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
380  OP_KETRMAX, and if so, to break the loop.  OP_KETRMAX, and if so, to break the loop.
381    
382    Possessive brackets
383    -------------------
384    
385    When a repeated group (capturing or non-capturing) is marked as possessive by
386    the "+" notation, e.g. (abc)++, different opcodes are used. Their names all
387    have POS on the end, e.g. OP_BRAPOS instead of OP_BRA and OP_SCPBRPOS instead
388    of OP_SCBRA. The end of such a group is marked by OP_KETRPOS. If the minimum
389    repetition is zero, the group is preceded by OP_BRAPOSZERO.
390    
391    
392  Assertions  Assertions
393  ----------  ----------
# Line 368  These are like other subpatterns, but th Line 417  These are like other subpatterns, but th
417  OP_SCOND for one that might match an empty string in an unbounded repeat. If  OP_SCOND for one that might match an empty string in an unbounded repeat. If
418  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
419  subpattern using the opcode OP_CREF followed by two bytes containing the  subpattern using the opcode OP_CREF followed by two bytes containing the
420  reference number. If the condition is "in recursion" (coded as "(?(R)"), or "in  reference number. OP_NCREF is used instead if the reference was generated by
421  recursion of group x" (coded as "(?(Rx)"), the group number is stored at the  name (so that the runtime code knows to check for duplicate names).
422  start of the subpattern using the opcode OP_RREF, and a value of zero for "the  
423  whole pattern". For a DEFINE condition, just the single byte OP_DEF is used (it  If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
424  has no associated data). Otherwise, a conditional subpattern always starts with  group x" (coded as "(?(Rx)"), the group number is stored at the start of the
425  one of the assertions.  subpattern using the opcode OP_RREF or OP_NRREF (cf OP_NCREF), and a value of
426    zero for "the whole pattern". For a DEFINE condition, just the single byte
427    OP_DEF is used (it has no associated data). Otherwise, a conditional subpattern
428    always starts with one of the assertions.
429    
430    
431  Recursion  Recursion
# Line 397  start of the following item, and another Line 449  start of the following item, and another
449  next item.  next item.
450    
451    
 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.  
   
452  Philip Hazel  Philip Hazel
453  August 2007  October 2011

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