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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 602 by ph10, Wed May 25 08:29:03 2011 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.
6    
7    
8  Historical note 1  Historical note 1
9  -----------------  -----------------
10    
# Line 13  not operate by backtracking, as the orig Line 17  not operate by backtracking, as the orig
17  Perl code does, but instead checked all possibilities simultaneously by keeping  Perl code does, but instead checked all possibilities simultaneously by keeping
18  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
19  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
20  algorithm". When the pattern was all used up, all remaining states were  algorithm", though it was not a traditional Finite State Machine (FSM). When
21  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
22  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
23  the pattern, as is expected in Unix and Perl-style regular expressions.  not necessarily maximize the individual wild portions of the pattern, as is
24    expected in Unix and Perl-style regular expressions.
25    
26    
27  Historical note 2  Historical note 2
28  -----------------  -----------------
29    
30  By contrast, the code originally written by Henry Spencer and subsequently  By contrast, the code originally written by Henry Spencer (which was
31  heavily modified for Perl actually compiles the expression twice: once in a  subsequently heavily modified for Perl) compiles the expression twice: once in
32  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
33  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
34  optionally, minimizing in Perl) the amount of the subject that matches  the original library.) The execution function operates by backtracking and
35  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
36  terminology.  matches individual wild portions of the pattern. This is an "NFA algorithm" in
37    Friedl's terminology.
38    
39    
40  OK, here's the real stuff  OK, here's the real stuff
41  -------------------------  -------------------------
# Line 37  unrelated to those mentioned above), I t Line 45  unrelated to those mentioned above), I t
45  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
46  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
47  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
48  first pass through the pattern is needed, for a number of reasons. PCRE works  first pass through the pattern is helpful for other reasons.
49  by running a very degenerate first pass to calculate a maximum store size, and  
50  then a second pass to do the real compile - which may use a bit less than the  
51  predicted amount of store. The idea is that this is going to turn out faster  Computing the memory requirement: how it was
52  because the first pass is degenerate and the second pass can just store stuff  --------------------------------------------
53  straight into the vector, which it knows is big enough. It does make the  
54  compiling functions bigger, of course, but they have got quite big anyway to  Up to and including release 6.7, PCRE worked by running a very degenerate first
55  handle all the Perl stuff.  pass to calculate a maximum store size, and then a second pass to do the real
56    compile - which might use a bit less than the predicted amount of memory. The
57    idea was that this would turn out faster than the Henry Spencer code because
58    the first pass is degenerate and the second pass can just store stuff straight
59    into the vector, which it knows is big enough.
60    
61    
62    Computing the memory requirement: how it is
63    -------------------------------------------
64    
65    By the time I was working on a potential 6.8 release, the degenerate first pass
66    had become very complicated and hard to maintain. Indeed one of the early
67    things I did for 6.8 was to fix Yet Another Bug in the memory computation. Then
68    I had a flash of inspiration as to how I could run the real compile function in
69    a "fake" mode that enables it to compute how much memory it would need, while
70    actually only ever using a few hundred bytes of working memory, and without too
71    many tests of the mode that might slow it down. So I refactored the compiling
72    functions to work this way. This got rid of about 600 lines of source. It
73    should make future maintenance and development easier. As this was such a major
74    change, I never released 6.8, instead upping the number to 7.0 (other quite
75    major changes were also present in the 7.0 release).
76    
77    A side effect of this work was that the previous limit of 200 on the nesting
78    depth of parentheses was removed. However, there is a downside: pcre_compile()
79    runs more slowly than before (30% or more, depending on the pattern) because it
80    is doing a full analysis of the pattern. My hope was that this would not be a
81    big issue, and in the event, nobody has commented on it.
82    
83    
84  Traditional matching function  Traditional matching function
85  -----------------------------  -----------------------------
86    
87  The "traditional", and original, matching function is called pcre_exec(), and  The "traditional", and original, matching function is called pcre_exec(), and
88  it implements an NFA algorithm, similar to the original Henry Spencer algorithm  it implements an NFA algorithm, similar to the original Henry Spencer algorithm
89  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
90  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
91  use most of the time.  will use most of the time.
92    
93    
94  Supplementary matching function  Supplementary matching function
95  -------------------------------  -------------------------------
# Line 63  pcre_dfa_exec(). This implements a DFA m Line 99  pcre_dfa_exec(). This implements a DFA m
99  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
100  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
101  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
102  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
103  same way. See the user documentation for details.  same way. See the user documentation for details.
104    
105    The algorithm that is used for pcre_dfa_exec() is not a traditional FSM,
106    because it may have a number of states active at one time. More work would be
107    needed at compile time to produce a traditional FSM where only one state is
108    ever active at once. I believe some other regex matchers work this way.
109    
110    
111    Changeable options
112    ------------------
113    
114    The /i, /m, or /s options (PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL) may
115    change in the middle of patterns. From PCRE 8.13, their processing is handled
116    entirely at compile time by generating different opcodes for the different
117    settings. The runtime functions do not need to keep track of an options state
118    any more.
119    
120    
121  Format of compiled patterns  Format of compiled patterns
122  ---------------------------  ---------------------------
123    
# Line 74  variable length. The first byte in an it Line 126  variable length. The first byte in an it
126  item is either implicit in the opcode or contained in the data bytes that  item is either implicit in the opcode or contained in the data bytes that
127  follow it.  follow it.
128    
129  In many cases below "two-byte" data values are specified. This is in fact just  In many cases below LINK_SIZE data values are specified for offsets within the
130  a default. PCRE can be compiled to use 3-byte or 4-byte values (impairing the  compiled pattern. The default value for LINK_SIZE is 2, but PCRE can be
131    compiled to use 3-byte or 4-byte values for these offsets (impairing the
132  performance). This is necessary only when patterns whose compiled length is  performance). This is necessary only when patterns whose compiled length is
133  greater than 64K are going to be processed. In this description, we assume the  greater than 64K are going to be processed. In this description, we assume the
134  "normal" compilation options.  "normal" compilation options. Data values that are counts (e.g. for
135    quantifiers) are always just two bytes long.
 A list of all the opcodes follows:  
136    
137  Opcodes with no following data  Opcodes with no following data
138  ------------------------------  ------------------------------
# Line 88  Opcodes with no following data Line 140  Opcodes with no following data
140  These items are all just one byte long  These items are all just one byte long
141    
142    OP_END                 end of pattern    OP_END                 end of pattern
143    OP_ANY                 match any character    OP_ANY                 match any one character other than newline
144      OP_ALLANY              match any one character, including newline
145    OP_ANYBYTE             match any single byte, even in UTF-8 mode    OP_ANYBYTE             match any single byte, even in UTF-8 mode
146    OP_SOD                 match start of data: \A    OP_SOD                 match start of data: \A
147    OP_SOM,                start of match (subject + offset): \G    OP_SOM,                start of match (subject + offset): \G
148    OP_CIRC                ^ (start of data, or after \n in multiline)    OP_SET_SOM,            set start of match (\K)
149      OP_CIRC                ^ (start of data)
150      OP_CIRCM               ^ multiline mode (start of data or after newline)
151    OP_NOT_WORD_BOUNDARY   \W    OP_NOT_WORD_BOUNDARY   \W
152    OP_WORD_BOUNDARY       \w    OP_WORD_BOUNDARY       \w
153    OP_NOT_DIGIT           \D    OP_NOT_DIGIT           \D
154    OP_DIGIT               \d    OP_DIGIT               \d
155      OP_NOT_HSPACE          \H
156      OP_HSPACE              \h
157    OP_NOT_WHITESPACE      \S    OP_NOT_WHITESPACE      \S
158    OP_WHITESPACE          \s    OP_WHITESPACE          \s
159      OP_NOT_VSPACE          \V
160      OP_VSPACE              \v
161    OP_NOT_WORDCHAR        \W    OP_NOT_WORDCHAR        \W
162    OP_WORDCHAR            \w    OP_WORDCHAR            \w
163    OP_EODN                match end of data or \n at end: \Z    OP_EODN                match end of data or \n at end: \Z
164    OP_EOD                 match end of data: \z    OP_EOD                 match end of data: \z
165    OP_DOLL                $ (end of data, or before \n in multiline)    OP_DOLL                $ (end of data, or before final newline)
166      OP_DOLLM               $ multiline mode (end of data or before newline)
167    OP_EXTUNI              match an extended Unicode character    OP_EXTUNI              match an extended Unicode character
168      OP_ANYNL               match any Unicode newline sequence
169    
170      OP_ACCEPT              ) These are Perl 5.10's "backtracking control
171      OP_COMMIT              ) verbs". If OP_ACCEPT is inside capturing
172      OP_FAIL                ) parentheses, it may be preceded by one or more
173      OP_PRUNE               ) OP_CLOSE, followed by a 2-byte number,
174      OP_SKIP                ) indicating which parentheses must be closed.
175    
176    
177    Backtracking control verbs with data
178    ------------------------------------
179    
180    OP_THEN is followed by a LINK_SIZE offset, which is the distance back to the
181    start of the current branch.
182    
183    OP_MARK is followed by the mark name, preceded by a one-byte length, and
184    followed by a binary zero. For (*PRUNE), (*SKIP), and (*THEN) with arguments,
185    the opcodes OP_PRUNE_ARG, OP_SKIP_ARG, and OP_THEN_ARG are used. For the first
186    two, the name follows immediately; for OP_THEN_ARG, it follows the LINK_SIZE
187    offset value.
188    
189    
190    Matching literal characters
191    ---------------------------
192    
193    The OP_CHAR opcode is followed by a single character that is to be matched
194    casefully. For caseless matching, OP_CHARI is used. In UTF-8 mode, the
195    character may be more than one byte long. (Earlier versions of PCRE used
196    multi-character strings, but this was changed to allow some new features to be
197    added.)
198    
199    
200  Repeating single characters  Repeating single characters
201  ---------------------------  ---------------------------
202    
203  The common repeats (*, +, ?) when applied to a single character use the  The common repeats (*, +, ?) when applied to a single character use the
204  following opcodes:  following opcodes, which come in caseful and caseless versions:
205    
206    OP_STAR    Caseful         Caseless
207    OP_MINSTAR    OP_STAR         OP_STARI
208    OP_PLUS    OP_MINSTAR      OP_MINSTARI
209    OP_MINPLUS    OP_POSSTAR      OP_POSSTARI
210    OP_QUERY    OP_PLUS         OP_PLUSI
211    OP_MINQUERY    OP_MINPLUS      OP_MINPLUSI
212      OP_POSPLUS      OP_POSPLUSI
213      OP_QUERY        OP_QUERYI
214      OP_MINQUERY     OP_MINQUERYI
215      OP_POSQUERY     OP_POSQUERYI
216    
217  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.
218  Those with "MIN" in their name are the minimizing versions. Each is followed by  Those with "MIN" in their name are the minimizing versions. Those with "POS" in
219  the character that is to be repeated. Other repeats make use of  their names are possessive versions. Each is followed by the character that is
220    to be repeated. Other repeats make use of these opcodes:
221    OP_UPTO  
222    OP_MINUPTO    Caseful         Caseless
223    OP_EXACT    OP_UPTO         OP_UPTOI
224      OP_MINUPTO      OP_MINUPTOI
225      OP_POSUPTO      OP_POSUPTOI
226      OP_EXACT        OP_EXACTI
227    
228  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
229  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
230  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
231  OP_UPTO (or OP_MINUPTO).  OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).
232    
233    
234  Repeating character types  Repeating character types
# Line 143  byte. The opcodes are: Line 240  byte. The opcodes are:
240    
241    OP_TYPESTAR    OP_TYPESTAR
242    OP_TYPEMINSTAR    OP_TYPEMINSTAR
243      OP_TYPEPOSSTAR
244    OP_TYPEPLUS    OP_TYPEPLUS
245    OP_TYPEMINPLUS    OP_TYPEMINPLUS
246      OP_TYPEPOSPLUS
247    OP_TYPEQUERY    OP_TYPEQUERY
248    OP_TYPEMINQUERY    OP_TYPEMINQUERY
249      OP_TYPEPOSQUERY
250    OP_TYPEUPTO    OP_TYPEUPTO
251    OP_TYPEMINUPTO    OP_TYPEMINUPTO
252      OP_TYPEPOSUPTO
253    OP_TYPEEXACT    OP_TYPEEXACT
254    
255    
# Line 157  Match by Unicode property Line 258  Match by Unicode property
258    
259  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
260  character by testing its Unicode property (the \p and \P escape sequences).  character by testing its Unicode property (the \p and \P escape sequences).
261  Each is followed by a single byte that encodes the desired property value.  Each is followed by two bytes that encode the desired property as a type and a
262    value.
263    
264  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
265  bytes: OP_PROP or OP_NOTPROP and then the desired property value.  three bytes: OP_PROP or OP_NOTPROP and then the desired property type and
266    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.)  
267    
268    
269  Character classes  Character classes
270  -----------------  -----------------
271    
272  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
273  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
274  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
275  values < 128, because OP_NOT is confined to single bytes.  with values < 128, because OP_NOT[I] is confined to single bytes.
276    
277  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
278  negated, single-character class. The normal ones (OP_STAR etc.) are used for a  repeated, negated, single-character class. The normal single-character opcodes
279  repeated positive single-character class.  (OP_STAR, etc.) are used for a repeated positive single-character class.
280    
281  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
282  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
283  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
284  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
285  significant end of each byte.  from the least significant end of each byte. In caseless mode, bits for both
286    cases are set.
287    
288  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,
289  subject characters with values greater than 256 can be handled correctly. For  subject characters with values greater than 256 can be handled correctly. For
290  OP_CLASS they don't match, whereas for OP_NCLASS they do.  OP_CLASS they do not match, whereas for OP_NCLASS they do.
291    
292  For classes containing characters with values > 255, OP_XCLASS is used. It  For classes containing characters with values > 255, OP_XCLASS is used. It
293  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
294  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
295  whether it's a positive or a negative class.  explicitly listed. There is a flag character than indicates whether it is a
296    positive or a negative class.
297    
298    
299  Back references  Back references
300  ---------------  ---------------
301    
302  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
303    reference number.
304    
305    
306  Repeating character classes and back references  Repeating character classes and back references
307  -----------------------------------------------  -----------------------------------------------
308    
309  Single-character classes are handled specially (see above). This applies to  Single-character classes are handled specially (see above). This section
310  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
311  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
312    if it is one of
313    
314    OP_CRSTAR    OP_CRSTAR
315    OP_CRMINSTAR    OP_CRMINSTAR
# Line 224  item. The matching code looks at the fol Line 321  item. The matching code looks at the fol
321    OP_CRMINRANGE    OP_CRMINRANGE
322    
323  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
324  four bytes of data, comprising the minimum and maximum repeat counts.  four bytes of data, comprising the minimum and maximum repeat counts. There are
325    no special possessive opcodes for these repeats; a possessive repeat is
326    compiled into an atomic group.
327    
328    
329  Brackets and alternation  Brackets and alternation
# Line 233  Brackets and alternation Line 332  Brackets and alternation
332  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
333  compile time, so alternation always happens in the context of brackets.  compile time, so alternation always happens in the context of brackets.
334    
335  Non-capturing brackets use the opcode OP_BRA, while capturing brackets use  [Note for North Americans: "bracket" to some English speakers, including
336  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.]
337  speakers, including myself, can be round, square, curly, or pointy. Hence this  
338  usage.]  Non-capturing brackets use the opcode OP_BRA. Originally PCRE was limited to 99
339    capturing brackets and it used a different opcode for each one. From release
340  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
341  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
342  first ones, up to EXTRACT_BASIC_MAX are handled with separate opcodes, as  this way, using the single opcode OP_CBRA.
 above. If there are more, the opcode is set to EXTRACT_BASIC_MAX+1, and the  
 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.)  
343    
344  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
345  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
346  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
347  the next one, or to the OP_KET opcode.  the next one, or to the OP_KET opcode. For capturing brackets, the bracket
348    number immediately follows the offset, always as a 2-byte item.
349    
350  OP_KET is used for subpatterns that do not repeat indefinitely, while  OP_KET is used for subpatterns that do not repeat indefinitely, while
351  OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or  OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
352  maximally respectively. All three are followed by LINK_SIZE bytes giving (as a  maximally respectively. All three are followed by LINK_SIZE bytes giving (as a
353  positive number) the offset back to the matching OP_BRA opcode.  positive number) the offset back to the matching bracket opcode.
354    
355  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
356  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
357  opcodes which tell the matcher that skipping this subpattern entirely is a  single-byte opcodes that tell the matcher that skipping the following
358  valid branch.  subpattern entirely is a valid branch. In the case of the first two, not
359    skipping the pattern is also valid (greedy and non-greedy). The third is used
360    when a pattern has the quantifier {0,0}. It cannot be entirely discarded,
361    because it may be called as a subroutine from elsewhere in the regex.
362    
363  A subpattern with an indefinite maximum repetition is replicated in the  A subpattern with an indefinite maximum repetition is replicated in the
364  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 368  as appropriate.
368  A subpattern with a bounded maximum repetition is replicated in a nested  A subpattern with a bounded maximum repetition is replicated in a nested
369  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
370  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
371  compiled as (abc)(abc)((abc)((abc)(abc)?)?)?.  compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group
372    has the same number.
373    
374    When a repeated subpattern has an unbounded upper limit, it is checked to see
375    whether it could match an empty string. If this is the case, the opcode in the
376    final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher
377    that it needs to check for matching an empty string when it hits OP_KETRMIN or
378    OP_KETRMAX, and if so, to break the loop.
379    
380    
381  Assertions  Assertions
# Line 286  each alternative of a lookbehind asserti Line 391  each alternative of a lookbehind asserti
391  fixed lengths.  fixed lengths.
392    
393    
394  Once-only subpatterns  Once-only (atomic) subpatterns
395  ---------------------  ------------------------------
396    
397  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
398  OP_ONCE.  OP_ONCE. The check for matching an empty string in an unbounded repeat is
399    handled entirely at runtime, so there is just this one opcode.
400    
401    
402  Conditional subpatterns  Conditional subpatterns
403  -----------------------  -----------------------
404    
405  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
406    OP_SCOND for one that might match an empty string in an unbounded repeat. If
407  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
408  subpattern using the opcode OP_CREF followed by two bytes containing the  subpattern using the opcode OP_CREF followed by two bytes containing the
409  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
410  same scheme is used, with a "reference number" of 0xffff. Otherwise, a  name (so that the runtime code knows to check for duplicate names).
411  conditional subpattern always starts with one of the assertions.  
412    If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
413    group x" (coded as "(?(Rx)"), the group number is stored at the start of the
414    subpattern using the opcode OP_RREF or OP_NRREF (cf OP_NCREF), and a value of
415    zero for "the whole pattern". For a DEFINE condition, just the single byte
416    OP_DEF is used (it has no associated data). Otherwise, a conditional subpattern
417    always starts with one of the assertions.
418    
419    
420  Recursion  Recursion
# Line 309  Recursion Line 422  Recursion
422    
423  Recursion either matches the current regex, or some subexpression. The opcode  Recursion either matches the current regex, or some subexpression. The opcode
424  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
425  from the start of the whole pattern.  from the start of the whole pattern. From release 6.5, OP_RECURSE is
426    automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
427    broke it). OP_RECURSE is also used for "subroutine" calls, even though they
428    are not strictly a recursion.
429    
430    
431  Callout  Callout
# Line 322  start of the following item, and another Line 438  start of the following item, and another
438  next item.  next item.
439    
440    
 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.  
   
441  Philip Hazel  Philip Hazel
442  March 2005  May 2011

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