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code/trunk/doc/Tech.Notes revision 77 by nigel, Sat Feb 24 21:40:45 2007 UTC code/trunk/HACKING revision 1204 by chpe, Mon Nov 5 16:33:05 2012 UTC
# Line 1  Line 1 
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. 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  -----------------  -----------------
11    
# Line 13  not operate by backtracking, as the orig Line 18  not operate by backtracking, as the orig
18  Perl code does, but instead checked all possibilities simultaneously by keeping  Perl code does, but instead checked all possibilities simultaneously by keeping
19  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
20  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
21  algorithm". When the pattern was all used up, all remaining states were  algorithm", though it was not a traditional Finite State Machine (FSM). When
22  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
23  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
24  the pattern, as is expected in Unix and Perl-style regular expressions.  not necessarily maximize the individual wild portions of the pattern, as is
25    expected in Unix and Perl-style regular expressions.
26    
27    
28  Historical note 2  Historical note 2
29  -----------------  -----------------
30    
31  By contrast, the code originally written by Henry Spencer and subsequently  By contrast, the code originally written by Henry Spencer (which was
32  heavily modified for Perl actually compiles the expression twice: once in a  subsequently heavily modified for Perl) compiles the expression twice: once in
33  dummy mode in order to find out how much store will be needed, and then for  a dummy mode in order to find out how much store will be needed, and then for
34  real. The execution function operates by backtracking and maximizing (or,  real. (The Perl version probably doesn't do this any more; I'm talking about
35  optionally, minimizing in Perl) the amount of the subject that matches  the original library.) The execution function operates by backtracking and
36  individual wild portions of the pattern. This is an "NFA algorithm" in Friedl's  maximizing (or, optionally, minimizing in Perl) the amount of the subject that
37  terminology.  matches individual wild portions of the pattern. This is an "NFA algorithm" in
38    Friedl's terminology.
39    
40    
41  OK, here's the real stuff  OK, here's the real stuff
42  -------------------------  -------------------------
# Line 37  unrelated to those mentioned above), I t Line 46  unrelated to those mentioned above), I t
46  that used an amount of store bounded by a multiple of the number of characters  that used an amount of store bounded by a multiple of the number of characters
47  in the pattern, to save on compiling time. However, because of the greater  in the pattern, to save on compiling time. However, because of the greater
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 needed, for a number of reasons. PCRE works  first pass through the pattern is helpful for other reasons.
50  by running a very degenerate first pass to calculate a maximum store size, and  
51  then a second pass to do the real compile - which may use a bit less than the  
52  predicted amount of store. The idea is that this is going to turn out faster  Support for 16-bit and 32-bit data strings
53  because the first pass is degenerate and the second pass can just store stuff  -------------------------------------------
54  straight into the vector, which it knows is big enough. It does make the  
55  compiling functions bigger, of course, but they have got quite big anyway to  From release 8.30, PCRE supports 16-bit as well as 8-bit data strings; and from
56  handle all the Perl stuff.  release 8.32, PCRE supports 32-bit data strings. The library can be compiled
57    in any combination of 8-bit, 16-bit or 32-bit modes, creating different
58    libraries. In the description that follows, the word "short" is
59    used for a 16-bit data quantity, and the word "unit" is used for a quantity
60    that is a byte in 8-bit mode, a short in 16-bit mode and a 32-bit unsigned
61    integer in 32-bit mode. However, so as not to over-complicate the text, the
62    names of PCRE functions are given in 8-bit form only.
63    
64    
65    Computing the memory requirement: how it was
66    --------------------------------------------
67    
68    Up to and including release 6.7, PCRE worked by running a very degenerate first
69    pass to calculate a maximum store size, and then a second pass to do the real
70    compile - which might use a bit less than the predicted amount of memory. The
71    idea was that this would turn out faster than the Henry Spencer code because
72    the first pass is degenerate and the second pass can just store stuff straight
73    into the vector, which it knows is big enough.
74    
75    
76    Computing the memory requirement: how it is
77    -------------------------------------------
78    
79    By the time I was working on a potential 6.8 release, the degenerate first pass
80    had become very complicated and hard to maintain. Indeed one of the early
81    things I did for 6.8 was to fix Yet Another Bug in the memory computation. Then
82    I had a flash of inspiration as to how I could run the real compile function in
83    a "fake" mode that enables it to compute how much memory it would need, while
84    actually only ever using a few hundred bytes of working memory, and without too
85    many tests of the mode that might slow it down. So I refactored the compiling
86    functions to work this way. This got rid of about 600 lines of source. It
87    should make future maintenance and development easier. As this was such a major
88    change, I never released 6.8, instead upping the number to 7.0 (other quite
89    major changes were also present in the 7.0 release).
90    
91    A side effect of this work was that the previous limit of 200 on the nesting
92    depth of parentheses was removed. However, there is a downside: pcre_compile()
93    runs more slowly than before (30% or more, depending on the pattern) because it
94    is doing a full analysis of the pattern. My hope was that this would not be a
95    big issue, and in the event, nobody has commented on it.
96    
97    
98  Traditional matching function  Traditional matching function
99  -----------------------------  -----------------------------
100    
101  The "traditional", and original, matching function is called pcre_exec(), and  The "traditional", and original, matching function is called pcre_exec(), and
102  it implements an NFA algorithm, similar to the original Henry Spencer algorithm  it implements an NFA algorithm, similar to the original Henry Spencer algorithm
103  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
104  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
105  use most of the time.  will use most of the time. From release 8.20, if PCRE is compiled with
106    just-in-time (JIT) support, and studying a compiled pattern with JIT is
107    successful, the JIT code is run instead of the normal pcre_exec() code, but the
108    result is the same.
109    
110    
111  Supplementary matching function  Supplementary matching function
112  -------------------------------  -------------------------------
# Line 63  pcre_dfa_exec(). This implements a DFA m Line 116  pcre_dfa_exec(). This implements a DFA m
116  simultaneously for all possible matches that start at one point in the subject  simultaneously for all possible matches that start at one point in the subject
117  string. (Going back to my roots: see Historical Note 1 above.) This function  string. (Going back to my roots: see Historical Note 1 above.) This function
118  intreprets the same compiled pattern data as pcre_exec(); however, not all the  intreprets the same compiled pattern data as pcre_exec(); however, not all the
119  facilities are available, and those that are don't always work in quite the  facilities are available, and those that are do not always work in quite the
120  same way. See the user documentation for details.  same way. See the user documentation for details.
121    
122    The algorithm that is used for pcre_dfa_exec() is not a traditional FSM,
123    because it may have a number of states active at one time. More work would be
124    needed at compile time to produce a traditional FSM where only one state is
125    ever active at once. I believe some other regex matchers work this way.
126    
127    
128    Changeable options
129    ------------------
130    
131    The /i, /m, or /s options (PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL) may
132    change in the middle of patterns. From PCRE 8.13, their processing is handled
133    entirely at compile time by generating different opcodes for the different
134    settings. The runtime functions do not need to keep track of an options state
135    any more.
136    
137    
138  Format of compiled patterns  Format of compiled patterns
139  ---------------------------  ---------------------------
140    
141  The compiled form of a pattern is a vector of bytes, containing items of  The compiled form of a pattern is a vector of units (bytes in 8-bit mode, or
142  variable length. The first byte in an item is an opcode, and the length of the  shorts in 16-bit mode, 32-bit unsigned integers in 32-bit mode), containing
143  item is either implicit in the opcode or contained in the data bytes that  items of variable length. The first unit in an item contains an opcode, and
144  follow it.  the length of the item is either implicit in the opcode or contained in the
145    data that follows it.
146  In many cases below "two-byte" data values are specified. This is in fact just  
147  a default. PCRE can be compiled to use 3-byte or 4-byte values (impairing the  In many cases listed below, LINK_SIZE data values are specified for offsets
148  performance). This is necessary only when patterns whose compiled length is  within the compiled pattern. LINK_SIZE always specifies a number of bytes. The
149  greater than 64K are going to be processed. In this description, we assume the  default value for LINK_SIZE is 2, but PCRE can be compiled to use 3-byte or
150  "normal" compilation options.  4-byte values for these offsets, although this impairs the performance. (3-byte
151    LINK_SIZE values are available only in 8-bit mode.) Specifing a LINK_SIZE
152  A list of all the opcodes follows:  larger than 2 is necessary only when patterns whose compiled length is greater
153    than 64K are going to be processed. In this description, we assume the "normal"
154    compilation options. Data values that are counts (e.g. for quantifiers) are
155    always just two bytes long (one short in 16-bit mode).
156    
157  Opcodes with no following data  Opcodes with no following data
158  ------------------------------  ------------------------------
159    
160  These items are all just one byte long  These items are all just one unit long
161    
162    OP_END                 end of pattern    OP_END                 end of pattern
163    OP_ANY                 match any character    OP_ANY                 match any one character other than newline
164      OP_ALLANY              match any one character, including newline
165    OP_ANYBYTE             match any single byte, even in UTF-8 mode    OP_ANYBYTE             match any single byte, even in UTF-8 mode
166    OP_SOD                 match start of data: \A    OP_SOD                 match start of data: \A
167    OP_SOM,                start of match (subject + offset): \G    OP_SOM,                start of match (subject + offset): \G
168    OP_CIRC                ^ (start of data, or after \n in multiline)    OP_SET_SOM,            set start of match (\K)
169      OP_CIRC                ^ (start of data)
170      OP_CIRCM               ^ multiline mode (start of data or after newline)
171    OP_NOT_WORD_BOUNDARY   \W    OP_NOT_WORD_BOUNDARY   \W
172    OP_WORD_BOUNDARY       \w    OP_WORD_BOUNDARY       \w
173    OP_NOT_DIGIT           \D    OP_NOT_DIGIT           \D
174    OP_DIGIT               \d    OP_DIGIT               \d
175      OP_NOT_HSPACE          \H
176      OP_HSPACE              \h
177    OP_NOT_WHITESPACE      \S    OP_NOT_WHITESPACE      \S
178    OP_WHITESPACE          \s    OP_WHITESPACE          \s
179      OP_NOT_VSPACE          \V
180      OP_VSPACE              \v
181    OP_NOT_WORDCHAR        \W    OP_NOT_WORDCHAR        \W
182    OP_WORDCHAR            \w    OP_WORDCHAR            \w
183    OP_EODN                match end of data or \n at end: \Z    OP_EODN                match end of data or \n at end: \Z
184    OP_EOD                 match end of data: \z    OP_EOD                 match end of data: \z
185    OP_DOLL                $ (end of data, or before \n in multiline)    OP_DOLL                $ (end of data, or before final newline)
186      OP_DOLLM               $ multiline mode (end of data or before newline)
187    OP_EXTUNI              match an extended Unicode character    OP_EXTUNI              match an extended Unicode character
188      OP_ANYNL               match any Unicode newline sequence
189    
190      OP_ACCEPT              ) These are Perl 5.10's "backtracking control
191      OP_COMMIT              ) verbs". If OP_ACCEPT is inside capturing
192      OP_FAIL                ) parentheses, it may be preceded by one or more
193      OP_PRUNE               ) OP_CLOSE, followed by a 2-byte number,
194      OP_SKIP                ) indicating which parentheses must be closed.
195    
196    
197    Backtracking control verbs with (optional) data
198    -----------------------------------------------
199    
200    (*THEN) without an argument generates the opcode OP_THEN and no following data.
201    OP_MARK is followed by the mark name, preceded by a one-unit length, and
202    followed by a binary zero. For (*PRUNE), (*SKIP), and (*THEN) with arguments,
203    the opcodes OP_PRUNE_ARG, OP_SKIP_ARG, and OP_THEN_ARG are used, with the name
204    following in the same format.
205    
206    
207    Matching literal characters
208    ---------------------------
209    
210    The OP_CHAR opcode is followed by a single character that is to be matched
211    casefully. For caseless matching, OP_CHARI is used. In UTF-8 or UTF-16 modes,
212    the character may be more than one unit long. In UTF-32 mode, characters
213    are always exactly one unit long.
214    
215    
216  Repeating single characters  Repeating single characters
217  ---------------------------  ---------------------------
218    
219  The common repeats (*, +, ?) when applied to a single character use the  The common repeats (*, +, ?), when applied to a single character, use the
220  following opcodes:  following opcodes, which come in caseful and caseless versions:
221    
222    OP_STAR    Caseful         Caseless
223    OP_MINSTAR    OP_STAR         OP_STARI
224    OP_PLUS    OP_MINSTAR      OP_MINSTARI
225    OP_MINPLUS    OP_POSSTAR      OP_POSSTARI
226    OP_QUERY    OP_PLUS         OP_PLUSI
227    OP_MINQUERY    OP_MINPLUS      OP_MINPLUSI
228      OP_POSPLUS      OP_POSPLUSI
229  In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.    OP_QUERY        OP_QUERYI
230  Those with "MIN" in their name are the minimizing versions. Each is followed by    OP_MINQUERY     OP_MINQUERYI
231  the character that is to be repeated. Other repeats make use of    OP_POSQUERY     OP_POSQUERYI
232    
233    OP_UPTO  Each opcode is followed by the character that is to be repeated. In ASCII mode,
234    OP_MINUPTO  these are two-unit items; in UTF-8 or UTF-16 modes, the length is variable; in
235    OP_EXACT  UTF-32 mode these are one-unit items.
236    Those with "MIN" in their names are the minimizing versions. Those with "POS"
237  which are followed by a two-byte count (most significant first) and the  in their names are possessive versions. Other repeats make use of these
238  repeated character. OP_UPTO matches from 0 to the given number. A repeat with a  opcodes:
239  non-zero minimum and a fixed maximum is coded as an OP_EXACT followed by an  
240  OP_UPTO (or OP_MINUPTO).    Caseful         Caseless
241      OP_UPTO         OP_UPTOI
242      OP_MINUPTO      OP_MINUPTOI
243      OP_POSUPTO      OP_POSUPTOI
244      OP_EXACT        OP_EXACTI
245    
246    Each of these is followed by a two-byte (one short) count (most significant
247    byte first in 8-bit mode) and then the repeated character. OP_UPTO matches from
248    0 to the given number. A repeat with a non-zero minimum and a fixed maximum is
249    coded as an OP_EXACT followed by an OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).
250    
251    
252  Repeating character types  Repeating character types
# Line 139  Repeating character types Line 254  Repeating character types
254    
255  Repeats of things like \d are done exactly as for single characters, except  Repeats of things like \d are done exactly as for single characters, except
256  that instead of a character, the opcode for the type is stored in the data  that instead of a character, the opcode for the type is stored in the data
257  byte. The opcodes are:  unit. The opcodes are:
258    
259    OP_TYPESTAR    OP_TYPESTAR
260    OP_TYPEMINSTAR    OP_TYPEMINSTAR
261      OP_TYPEPOSSTAR
262    OP_TYPEPLUS    OP_TYPEPLUS
263    OP_TYPEMINPLUS    OP_TYPEMINPLUS
264      OP_TYPEPOSPLUS
265    OP_TYPEQUERY    OP_TYPEQUERY
266    OP_TYPEMINQUERY    OP_TYPEMINQUERY
267      OP_TYPEPOSQUERY
268    OP_TYPEUPTO    OP_TYPEUPTO
269    OP_TYPEMINUPTO    OP_TYPEMINUPTO
270      OP_TYPEPOSUPTO
271    OP_TYPEEXACT    OP_TYPEEXACT
272    
273    
# Line 157  Match by Unicode property Line 276  Match by Unicode property
276    
277  OP_PROP and OP_NOTPROP are used for positive and negative matches of a  OP_PROP and OP_NOTPROP are used for positive and negative matches of a
278  character by testing its Unicode property (the \p and \P escape sequences).  character by testing its Unicode property (the \p and \P escape sequences).
279  Each is followed by a single byte that encodes the desired property value.  Each is followed by two units that encode the desired property as a type and a
280    value.
281    
282  Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by two  Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by
283  bytes: OP_PROP or OP_NOTPROP and then the desired property value.  three units: OP_PROP or OP_NOTPROP, and then the desired property type and
284    value.
   
 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.)  
285    
286    
287  Character classes  Character classes
288  -----------------  -----------------
289    
290  If there is only one character, OP_CHAR or OP_CHARNC is used for a positive  If there is only one character in the class, OP_CHAR or OP_CHARI is used for a
291  class, and OP_NOT for a negative one (that is, for something like [^a]).  positive class, and OP_NOT or OP_NOTI for a negative one (that is, for
292  However, in UTF-8 mode, the use of OP_NOT applies only to characters with  something like [^a]).
293  values < 128, because OP_NOT is confined to single bytes.  
294    Another set of 13 repeating opcodes (called OP_NOTSTAR etc.) are used for
295  Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated,  repeated, negated, single-character classes. The normal single-character
296  negated, single-character class. The normal ones (OP_STAR etc.) are used for a  opcodes (OP_STAR, etc.) are used for repeated positive single-character
297  repeated positive single-character class.  classes.
298    
299  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
300  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
301  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 (16-short)
302  bit for every character that is acceptable. The bits are counted from the least  bit map containing a 1 bit for every character that is acceptable. The bits are
303  significant end of each byte.  counted from the least significant end of each unit. In caseless mode, bits for
304    both cases are set.
305  The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,  
306  subject characters with values greater than 256 can be handled correctly. For  The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8/16/32 mode,
307  OP_CLASS they don't match, whereas for OP_NCLASS they do.  subject characters with values greater than 255 can be handled correctly. For
308    OP_CLASS they do not match, whereas for OP_NCLASS they do.
309  For classes containing characters with values > 255, OP_XCLASS is used. It  
310  optionally uses a bit map (if any characters lie within it), followed by a list  For classes containing characters with values greater than 255, OP_XCLASS is
311  of pairs and single characters. There is a flag character than indicates  used. It optionally uses a bit map (if any characters lie within it), followed
312  whether it's a positive or a negative class.  by a list of pairs (for a range) and single characters. In caseless mode, both
313    cases are explicitly listed. There is a flag character than indicates whether
314    it is a positive or a negative class.
315    
316    
317  Back references  Back references
318  ---------------  ---------------
319    
320  OP_REF is followed by two bytes containing the reference number.  OP_REF (caseful) or OP_REFI (caseless) is followed by two bytes (one short)
321    containing the reference number.
322    
323    
324  Repeating character classes and back references  Repeating character classes and back references
325  -----------------------------------------------  -----------------------------------------------
326    
327  Single-character classes are handled specially (see above). This applies to  Single-character classes are handled specially (see above). This section
328  OP_CLASS and OP_REF. In both cases, the repeat information follows the base  applies to OP_CLASS and OP_REF[I]. In both cases, the repeat information
329  item. The matching code looks at the following opcode to see if it is one of  follows the base item. The matching code looks at the following opcode to see
330    if it is one of
331    
332    OP_CRSTAR    OP_CRSTAR
333    OP_CRMINSTAR    OP_CRMINSTAR
# Line 223  item. The matching code looks at the fol Line 338  item. The matching code looks at the fol
338    OP_CRRANGE    OP_CRRANGE
339    OP_CRMINRANGE    OP_CRMINRANGE
340    
341  All but the last two are just single-byte items. The others are followed by  All but the last two are just single-unit items. The others are followed by
342  four bytes of data, comprising the minimum and maximum repeat counts.  four bytes (two shorts) of data, comprising the minimum and maximum repeat
343    counts. There are no special possessive opcodes for these repeats; a possessive
344    repeat is compiled into an atomic group.
345    
346    
347  Brackets and alternation  Brackets and alternation
# Line 233  Brackets and alternation Line 350  Brackets and alternation
350  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
351  compile time, so alternation always happens in the context of brackets.  compile time, so alternation always happens in the context of brackets.
352    
353  Non-capturing brackets use the opcode OP_BRA, while capturing brackets use  [Note for North Americans: "bracket" to some English speakers, including
354  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 rather than
355  speakers, including myself, can be round, square, curly, or pointy. Hence this  "parentheses".]
356  usage.]  
357    Non-capturing brackets use the opcode OP_BRA. Originally PCRE was limited to 99
358  Originally PCRE was limited to 99 capturing brackets (so as not to use up all  capturing brackets and it used a different opcode for each one. From release
359  the opcodes). From release 3.5, there is no limit. What happens is that the  3.5, the limit was removed by putting the bracket number into the data for
360  first ones, up to EXTRACT_BASIC_MAX are handled with separate opcodes, as  higher-numbered brackets. From release 7.0 all capturing brackets are handled
361  above. If there are more, the opcode is set to EXTRACT_BASIC_MAX+1, and the  this way, using the single opcode OP_CBRA.
 first operation in the bracket is OP_BRANUMBER, followed by a 2-byte bracket  
 number. This opcode is ignored while matching, but is fished out when handling  
 the bracket itself. (They could have all been done like this, but I was making  
 minimal changes.)  
362    
363  A bracket opcode is followed by LINK_SIZE bytes which give the offset to the  A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
364  next alternative OP_ALT or, if there aren't any branches, to the matching  next alternative OP_ALT or, if there aren't any branches, to the matching
365  OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to  OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
366  the next one, or to the OP_KET opcode.  the next one, or to the OP_KET opcode. For capturing brackets, the bracket
367    number immediately follows the offset, always as a 2-byte (one short) item.
368    
369  OP_KET is used for subpatterns that do not repeat indefinitely, while  OP_KET is used for subpatterns that do not repeat indefinitely, and
370  OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or  OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
371  maximally respectively. All three are followed by LINK_SIZE bytes giving (as a  maximally respectively (see below for possessive repetitions). All three are
372  positive number) the offset back to the matching OP_BRA opcode.  followed by LINK_SIZE bytes giving (as a positive number) the offset back to
373    the matching bracket opcode.
374    
375  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
376  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
377  opcodes which tell the matcher that skipping this subpattern entirely is a  single-unit opcodes that tell the matcher that skipping the following
378  valid branch.  subpattern entirely is a valid branch. In the case of the first two, not
379    skipping the pattern is also valid (greedy and non-greedy). The third is used
380    when a pattern has the quantifier {0,0}. It cannot be entirely discarded,
381    because it may be called as a subroutine from elsewhere in the regex.
382    
383  A subpattern with an indefinite maximum repetition is replicated in the  A subpattern with an indefinite maximum repetition is replicated in the
384  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 270  as appropriate. Line 388  as appropriate.
388  A subpattern with a bounded maximum repetition is replicated in a nested  A subpattern with a bounded maximum repetition is replicated in a nested
389  fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO  fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
390  before 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
391  compiled as (abc)(abc)((abc)((abc)(abc)?)?)?.  compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group
392    has the same number.
393    
394    When a repeated subpattern has an unbounded upper limit, it is checked to see
395    whether it could match an empty string. If this is the case, the opcode in the
396    final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher
397    that it needs to check for matching an empty string when it hits OP_KETRMIN or
398    OP_KETRMAX, and if so, to break the loop.
399    
400    Possessive brackets
401    -------------------
402    
403    When a repeated group (capturing or non-capturing) is marked as possessive by
404    the "+" notation, e.g. (abc)++, different opcodes are used. Their names all
405    have POS on the end, e.g. OP_BRAPOS instead of OP_BRA and OP_SCPBRPOS instead
406    of OP_SCBRA. The end of such a group is marked by OP_KETRPOS. If the minimum
407    repetition is zero, the group is preceded by OP_BRAPOSZERO.
408    
409    
410  Assertions  Assertions
# Line 279  Assertions Line 413  Assertions
413  Forward assertions are just like other subpatterns, but starting with one of  Forward assertions are just like other subpatterns, but starting with one of
414  the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes  the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
415  OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion  OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
416  is OP_REVERSE, followed by a two byte count of the number of characters to move  is OP_REVERSE, followed by a two byte (one short) count of the number of
417  back the pointer in the subject string. When operating in UTF-8 mode, the count  characters to move back the pointer in the subject string. In ASCII mode, the
418  is a character count rather than a byte count. A separate count is present in  count is a number of units, but in UTF-8/16 mode each character may occupy more
419  each alternative of a lookbehind assertion, allowing them to have different  than one unit; in UTF-32 mode each character occupies exactly one unit.
420  fixed lengths.  A separate count is present in each alternative of a lookbehind
421    assertion, allowing them to have different fixed lengths.
422    
423    
424  Once-only subpatterns  Once-only (atomic) subpatterns
425  ---------------------  ------------------------------
426    
427  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
428  OP_ONCE.  OP_ONCE. The check for matching an empty string in an unbounded repeat is
429    handled entirely at runtime, so there is just this one opcode.
430    
431    
432  Conditional subpatterns  Conditional subpatterns
433  -----------------------  -----------------------
434    
435  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
436    OP_SCOND for one that might match an empty string in an unbounded repeat. If
437  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
438  subpattern using the opcode OP_CREF followed by two bytes containing the  subpattern using the opcode OP_CREF followed by two bytes (one short)
439  reference number. If the condition is "in recursion" (coded as "(?(R)"), the  containing the reference number. OP_NCREF is used instead if the reference was
440  same scheme is used, with a "reference number" of 0xffff. Otherwise, a  generated by name (so that the runtime code knows to check for duplicate
441  conditional subpattern always starts with one of the assertions.  names).
442    
443    If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
444    group x" (coded as "(?(Rx)"), the group number is stored at the start of the
445    subpattern using the opcode OP_RREF or OP_NRREF (cf OP_NCREF), and a value of
446    zero for "the whole pattern". For a DEFINE condition, just the single unit
447    OP_DEF is used (it has no associated data). Otherwise, a conditional subpattern
448    always starts with one of the assertions.
449    
450    
451  Recursion  Recursion
# Line 309  Recursion Line 453  Recursion
453    
454  Recursion either matches the current regex, or some subexpression. The opcode  Recursion either matches the current regex, or some subexpression. The opcode
455  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
456  from the start of the whole pattern.  from the start of the whole pattern. From release 6.5, OP_RECURSE is
457    automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
458    broke it). OP_RECURSE is also used for "subroutine" calls, even though they
459    are not strictly a recursion.
460    
461    
462  Callout  Callout
463  -------  -------
464    
465  OP_CALLOUT is followed by one byte of data that holds a callout number in the  OP_CALLOUT is followed by one unit of data that holds a callout number in the
466  range 0 to 254 for manual callouts, or 255 for an automatic callout. In both  range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
467  cases there follows a two-byte value giving the offset in the pattern to the  cases there follows a two-byte (one short) value giving the offset in the
468  start of the following item, and another two-byte item giving the length of the  pattern to the start of the following item, and another two-byte (one short)
469  next item.  item giving the length of the next item.
470    
   
 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.  
471    
472  Philip Hazel  Philip Hazel
473  March 2005  February 2012

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