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code/trunk/doc/Tech.Notes revision 71 by nigel, Sat Feb 24 21:40:24 2007 UTC code/trunk/HACKING revision 612 by ph10, Sat Jul 2 15:20:59 2011 UTC
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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
10    -----------------
11    
12  Many years ago I implemented some regular expression functions to an algorithm  Many years ago I implemented some regular expression functions to an algorithm
13  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
14  restricted in what they could do by comparison with Perl. The interesting part  restricted in what they could do by comparison with Perl. The interesting part
# Line 9  form of an expression was known in advan Line 17  form of an expression was known in advan
17  not operate by backtracking, as the original Henry Spencer code and current  not operate by backtracking, as the original Henry Spencer code and current
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  By contrast, the code originally written by Henry Spencer and subsequently  
27  heavily modified for Perl actually compiles the expression twice: once in a  
28  dummy mode in order to find out how much store will be needed, and then for  Historical note 2
29  real. The execution function operates by backtracking and maximizing (or,  -----------------
30  optionally, minimizing in Perl) the amount of the subject that matches  
31  individual wild portions of the pattern. This is an "NFA algorithm" in Friedl's  By contrast, the code originally written by Henry Spencer (which was
32  terminology.  subsequently heavily modified for Perl) compiles the expression twice: once in
33    a dummy mode in order to find out how much store will be needed, and then for
34  For the set of functions that forms PCRE (which are unrelated to those  real. (The Perl version probably doesn't do this any more; I'm talking about
35  mentioned above), I tried at first to invent an algorithm that used an amount  the original library.) The execution function operates by backtracking and
36  of store bounded by a multiple of the number of characters in the pattern, to  maximizing (or, optionally, minimizing in Perl) the amount of the subject that
37  save on compiling time. However, because of the greater complexity in Perl  matches individual wild portions of the pattern. This is an "NFA algorithm" in
38  regular expressions, I couldn't do this. In any case, a first pass through the  Friedl's terminology.
39  pattern is needed, for a number of reasons. PCRE works by running a very  
40  degenerate first pass to calculate a maximum store size, and then a second pass  
41  to do the real compile - which may use a bit less than the predicted amount of  OK, here's the real stuff
42  store. The idea is that this is going to turn out faster because the first pass  -------------------------
43  is degenerate and the second pass can just store stuff straight into the  
44  vector. It does make the compiling functions bigger, of course, but they have  For the set of functions that form the "basic" PCRE library (which are
45  got quite big anyway to handle all the Perl stuff.  unrelated to those mentioned above), I tried at first to invent an algorithm
46    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
48    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.
50    
51    
52    Computing the memory requirement: how it was
53    --------------------------------------------
54    
55    Up to and including release 6.7, PCRE worked by running a very degenerate first
56    pass to calculate a maximum store size, and then a second pass to do the real
57    compile - which might use a bit less than the predicted amount of memory. The
58    idea was that this would turn out faster than the Henry Spencer code because
59    the first pass is degenerate and the second pass can just store stuff straight
60    into the vector, which it knows is big enough.
61    
62    
63    Computing the memory requirement: how it is
64    -------------------------------------------
65    
66    By the time I was working on a potential 6.8 release, the degenerate first pass
67    had become very complicated and hard to maintain. Indeed one of the early
68    things I did for 6.8 was to fix Yet Another Bug in the memory computation. Then
69    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
71    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 refactored the compiling
73    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
75    change, I never released 6.8, instead upping the number to 7.0 (other quite
76    major changes were also present in the 7.0 release).
77    
78    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()
80    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 was that this would not be a
82    big issue, and in the event, nobody has commented on it.
83    
84    
85    Traditional matching function
86    -----------------------------
87    
88    The "traditional", and original, matching function is called pcre_exec(), and
89    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
91    as compatible with Perl as possible. This is the function most users of PCRE
92    will use most of the time.
93    
94    
95    Supplementary matching function
96    -------------------------------
97    
98    From PCRE 6.0, there is also a supplementary matching function called
99    pcre_dfa_exec(). This implements a DFA matching algorithm that searches
100    simultaneously for all possible matches that start at one point in the subject
101    string. (Going back to my roots: see Historical Note 1 above.) This function
102    intreprets the same compiled pattern data as pcre_exec(); however, not all the
103    facilities are available, and those that are do not always work in quite the
104    same way. See the user documentation for details.
105    
106    The algorithm that is used for pcre_dfa_exec() is not a traditional FSM,
107    because it may have a number of states active at one time. More work would be
108    needed at compile time to produce a traditional FSM where only one state is
109    ever active at once. I believe some other regex matchers work this way.
110    
111    
112    Changeable options
113    ------------------
114    
115    The /i, /m, or /s options (PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL) may
116    change in the middle of patterns. From PCRE 8.13, their processing is handled
117    entirely at compile time by generating different opcodes for the different
118    settings. The runtime functions do not need to keep track of an options state
119    any more.
120    
121    
122    Format of compiled patterns
123    ---------------------------
124    
125  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
126  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
127  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
128  follow it. A list of all the opcodes follows:  follow it.
129    
130    In many cases below LINK_SIZE data values are specified for offsets within the
131    compiled pattern. The default value for LINK_SIZE is 2, but PCRE can be
132    compiled to use 3-byte or 4-byte values for these offsets (impairing the
133    performance). This is necessary only when patterns whose compiled length is
134    greater than 64K are going to be processed. In this description, we assume the
135    "normal" compilation options. Data values that are counts (e.g. for
136    quantifiers) are always just two bytes long.
137    
138  Opcodes with no following data  Opcodes with no following data
139  ------------------------------  ------------------------------
# Line 47  Opcodes with no following data Line 141  Opcodes with no following data
141  These items are all just one byte long  These items are all just one byte long
142    
143    OP_END                 end of pattern    OP_END                 end of pattern
144    OP_ANY                 match any character    OP_ANY                 match any one character other than newline
145    OP_ANYBYTE             match any single byte, even in UTF-8 mode    OP_ALLANY              match any one character, including newline
146      OP_ANYBYTE             match any single byte, even in UTF-8 mode
147    OP_SOD                 match start of data: \A    OP_SOD                 match start of data: \A
148    OP_SOM,                start of match (subject + offset): \G    OP_SOM,                start of match (subject + offset): \G
149    OP_CIRC                ^ (start of data, or after \n in multiline)    OP_SET_SOM,            set start of match (\K)
150      OP_CIRC                ^ (start of data)
151      OP_CIRCM               ^ multiline mode (start of data or after newline)
152    OP_NOT_WORD_BOUNDARY   \W    OP_NOT_WORD_BOUNDARY   \W
153    OP_WORD_BOUNDARY       \w    OP_WORD_BOUNDARY       \w
154    OP_NOT_DIGIT           \D    OP_NOT_DIGIT           \D
155    OP_DIGIT               \d    OP_DIGIT               \d
156      OP_NOT_HSPACE          \H
157      OP_HSPACE              \h
158    OP_NOT_WHITESPACE      \S    OP_NOT_WHITESPACE      \S
159    OP_WHITESPACE          \s    OP_WHITESPACE          \s
160      OP_NOT_VSPACE          \V
161      OP_VSPACE              \v
162    OP_NOT_WORDCHAR        \W    OP_NOT_WORDCHAR        \W
163    OP_WORDCHAR            \w    OP_WORDCHAR            \w
164    OP_EODN                match end of data or \n at end: \Z    OP_EODN                match end of data or \n at end: \Z
165    OP_EOD                 match end of data: \z    OP_EOD                 match end of data: \z
166    OP_DOLL                $ (end of data, or before \n in multiline)    OP_DOLL                $ (end of data, or before final newline)
167      OP_DOLLM               $ multiline mode (end of data or before newline)
168      OP_EXTUNI              match an extended Unicode character
169      OP_ANYNL               match any Unicode newline sequence
170    
171      OP_ACCEPT              ) These are Perl 5.10's "backtracking control
172      OP_COMMIT              ) verbs". If OP_ACCEPT is inside capturing
173      OP_FAIL                ) parentheses, it may be preceded by one or more
174      OP_PRUNE               ) OP_CLOSE, followed by a 2-byte number,
175      OP_SKIP                ) indicating which parentheses must be closed.
176    
177    
178    Backtracking control verbs with data
179    ------------------------------------
180    
181    OP_THEN is followed by a LINK_SIZE offset, which is the distance back to the
182    start of the current branch.
183    
184    OP_MARK is followed by the mark name, preceded by a one-byte length, and
185    followed by a binary zero. For (*PRUNE), (*SKIP), and (*THEN) with arguments,
186    the opcodes OP_PRUNE_ARG, OP_SKIP_ARG, and OP_THEN_ARG are used. For the first
187    two, the name follows immediately; for OP_THEN_ARG, it follows the LINK_SIZE
188    offset value.
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 appear as  The common repeats (*, +, ?) when applied to a single character use the
205  two-byte items using the 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_PLUS    OP_MINSTAR      OP_MINSTARI
210    OP_MINPLUS    OP_POSSTAR      OP_POSSTARI
211    OP_QUERY    OP_PLUS         OP_PLUSI
212    OP_MINQUERY    OP_MINPLUS      OP_MINPLUSI
213      OP_POSPLUS      OP_POSPLUSI
214  Those with "MIN" in their name are the minimizing versions. Each is followed by    OP_QUERY        OP_QUERYI
215  the character that is to be repeated. Other repeats make use of    OP_MINQUERY     OP_MINQUERYI
216      OP_POSQUERY     OP_POSQUERYI
217    OP_UPTO  
218    OP_MINUPTO  In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.
219    OP_EXACT  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
221    to be repeated. Other repeats make use of these opcodes:
222    
223      Caseful         Caseless
224      OP_UPTO         OP_UPTOI
225      OP_MINUPTO      OP_MINUPTOI
226      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).  OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).
233    
234    
235  Repeating character types  Repeating character types
# Line 100  byte. The opcodes are: Line 241  byte. The opcodes are:
241    
242    OP_TYPESTAR    OP_TYPESTAR
243    OP_TYPEMINSTAR    OP_TYPEMINSTAR
244      OP_TYPEPOSSTAR
245    OP_TYPEPLUS    OP_TYPEPLUS
246    OP_TYPEMINPLUS    OP_TYPEMINPLUS
247      OP_TYPEPOSPLUS
248    OP_TYPEQUERY    OP_TYPEQUERY
249    OP_TYPEMINQUERY    OP_TYPEMINQUERY
250      OP_TYPEPOSQUERY
251    OP_TYPEUPTO    OP_TYPEUPTO
252    OP_TYPEMINUPTO    OP_TYPEMINUPTO
253      OP_TYPEPOSUPTO
254    OP_TYPEEXACT    OP_TYPEEXACT
255    
256    
257  Matching a character string  Match by Unicode property
258  ---------------------------  -------------------------
259    
260  The OP_CHARS opcode is followed by a one-byte count and then that number of  OP_PROP and OP_NOTPROP are used for positive and negative matches of a
261  characters. If there are more than 255 characters in sequence, successive  character by testing its Unicode property (the \p and \P escape sequences).
262  instances of OP_CHARS are used.  Each is followed by two bytes that encode the desired property as a type and a
263    value.
264    
265    Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by
266    three bytes: OP_PROP or OP_NOTPROP and then the desired property type and
267    value.
268    
269    
270  Character classes  Character classes
271  -----------------  -----------------
272    
273  If there is only one character, OP_CHARS is used for a positive class,  If there is only one character, OP_CHAR or OP_CHARI is used for a positive
274  and OP_NOT for a negative one (that is, for something like [^a]). However, in  class, and OP_NOT or OP_NOTI for a negative one (that is, for something like
275  UTF-8 mode, this applies only to characters with values < 128, because OP_NOT  [^a]). However, in UTF-8 mode, the use of OP_NOT[I] applies only to characters
276  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  The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,  
289  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 mode,
290  OP_CLASS they don't match, whereas for OP_NCLASS they do.  subject characters with values greater than 256 can be handled correctly. For
291    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 applies to  Single-character classes are handled specially (see above). This section
311  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
312  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
313    if it is one of
314    
315    OP_CRSTAR    OP_CRSTAR
316    OP_CRMINSTAR    OP_CRMINSTAR
# Line 168  item. The matching code looks at the fol Line 322  item. The matching code looks at the fol
322    OP_CRMINRANGE    OP_CRMINRANGE
323    
324  All but the last two are just single-byte items. The others are followed by  All but the last two are just single-byte items. The others are followed by
325  four bytes of data, comprising the minimum and maximum repeat counts.  four bytes of data, comprising the minimum and maximum repeat counts. There are
326    no special possessive opcodes for these repeats; a possessive repeat is
327    compiled into an atomic group.
328    
329    
330  Brackets and alternation  Brackets and alternation
# Line 177  Brackets and alternation Line 333  Brackets and alternation
333  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
334  compile time, so alternation always happens in the context of brackets.  compile time, so alternation always happens in the context of brackets.
335    
336  Non-capturing brackets use the opcode OP_BRA, while capturing brackets use  [Note for North Americans: "bracket" to some English speakers, including
337  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.]
338  speakers, including myself, can be round, square, curly, or pointy. Hence this  
339  usage.]  Non-capturing brackets use the opcode OP_BRA. Originally PCRE was limited to 99
340    capturing brackets and it used a different opcode for each one. From release
341  Originally PCRE was limited to 99 capturing brackets (so as not to use up all  3.5, the limit was removed by putting the bracket number into the data for
342  the opcodes). From release 3.5, there is no limit. What happens is that the  higher-numbered brackets. From release 7.0 all capturing brackets are handled
343  first ones, up to EXTRACT_BASIC_MAX are handled with separate opcodes, as  this way, using the single opcode OP_CBRA.
344  above. If there are more, the opcode is set to EXTRACT_BASIC_MAX+1, and the  
345  first operation in the bracket is OP_BRANUMBER, followed by a 2-byte bracket  A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
346  number. This opcode is ignored while matching, but is fished out when handling  next alternative OP_ALT or, if there aren't any branches, to the matching
347  the bracket itself. (They could have all been done like this, but I was making  OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
348  minimal changes.)  the next one, or to the OP_KET opcode. For capturing brackets, the bracket
349    number immediately follows the offset, always as a 2-byte item.
 A bracket opcode is followed by two bytes which give the offset to the next  
 alternative OP_ALT or, if there aren't any branches, to the matching KET  
 opcode. Each OP_ALT is followed by two bytes giving the offset to the next one,  
 or to the KET opcode.  
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 two bytes giving (as a  maximally respectively (see below for possessive repetitions). All three are
354  positive number) the offset back to the matching BRA 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 a BRAZERO if the  compiled data its minimum number of times (or once with OP_BRAZERO if the
367  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
368  appropriate.  as appropriate.
369    
370  A subpattern with a bounded maximum repetition is replicated in a nested  A subpattern with a bounded maximum repetition is replicated in a nested
371  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
372  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
373  compiled as (abc)(abc)((abc)((abc)(abc)?)?)?. The 99 and 200 bracket limits do  compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group
374  not apply to these internally generated brackets.  has the same number.
375    
376    When a repeated subpattern has an unbounded upper limit, it is checked to see
377    whether it could match an empty string. If this is the case, the opcode in the
378    final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher
379    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.
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
# Line 231  each alternative of a lookbehind asserti Line 402  each alternative of a lookbehind asserti
402  fixed lengths.  fixed lengths.
403    
404    
405  Once-only subpatterns  Once-only (atomic) subpatterns
406  ---------------------  ------------------------------
407    
408  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
409  OP_ONCE.  OP_ONCE. The check for matching an empty string in an unbounded repeat is
410    handled entirely at runtime, so there is just this one opcode.
411    
412    
413  Conditional subpatterns  Conditional subpatterns
414  -----------------------  -----------------------
415    
416  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
417    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)"), the  reference number. OP_NCREF is used instead if the reference was generated by
421  same scheme is used, with a "reference number" of 0xffff. Otherwise, a  name (so that the runtime code knows to check for duplicate names).
422  conditional subpattern always starts with one of the assertions.  
423    If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
424    group x" (coded as "(?(Rx)"), the group number is stored at the start of the
425    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 254  Recursion Line 433  Recursion
433    
434  Recursion either matches the current regex, or some subexpression. The opcode  Recursion either matches the current regex, or some subexpression. The opcode
435  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
436  from the start of the whole pattern.  from the start of the whole pattern. From release 6.5, OP_RECURSE is
437    automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
438    broke it). OP_RECURSE is also used for "subroutine" calls, even though they
439    are not strictly a recursion.
440    
441    
442  Callout  Callout
443  -------  -------
444    
445  OP_CALLOUT is followed by one byte of data that holds a callout number in the  OP_CALLOUT is followed by one byte of data that holds a callout number in the
446  range 0 to 255.  range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
447    cases there follows a two-byte value giving the offset in the pattern to the
448    start of the following item, and another two-byte item giving the length of the
449  Changing options  next item.
 ----------------  
450    
 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.  
451    
452  Philip Hazel  Philip Hazel
453  August 2003  July 2011

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