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code/trunk/doc/Tech.Notes revision 49 by nigel, Sat Feb 24 21:39:33 2007 UTC code/trunk/HACKING revision 460 by ph10, Sun Oct 4 09:27:20 2009 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.
6    
7    Historical note 1
8    -----------------
9    
10  Many years ago I implemented some regular expression functions to an algorithm  Many years ago I implemented some regular expression functions to an algorithm
11  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
12  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
13  about the algorithm was that the amount of space required to hold the compiled  about the algorithm was that the amount of space required to hold the compiled
14  form of an expression was known in advance. The code to apply an expression did  form of an expression was known in advance. The code to apply an expression did
15  not operate by backtracking, as the Henry Spencer and Perl code does, but  not operate by backtracking, as the original Henry Spencer code and current
16  instead checked all possibilities simultaneously by keeping a list of current  Perl code does, but instead checked all possibilities simultaneously by keeping
17  states and checking all of them as it advanced through the subject string. (In  a list of current states and checking all of them as it advanced through the
18  the terminology of Jeffrey Friedl's book, it was a "DFA algorithm".) When the  subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA
19  pattern was all used up, all remaining states were possible matches, and the  algorithm", though it was not a traditional Finite State Machine (FSM). When
20  one matching the longest subset of the subject string was chosen. This did not  the pattern was all used up, all remaining states were possible matches, and
21  necessarily maximize the individual wild portions of the pattern, as is  the one matching the longest subset of the subject string was chosen. This did
22    not necessarily maximize the individual wild portions of the pattern, as is
23  expected in Unix and Perl-style regular expressions.  expected in Unix and Perl-style regular expressions.
24    
25  By contrast, the code originally written by Henry Spencer and subsequently  Historical note 2
26  heavily modified for Perl actually compiles the expression twice: once in a  -----------------
27  dummy mode in order to find out how much store will be needed, and then for  
28  real. The execution function operates by backtracking and maximizing (or,  By contrast, the code originally written by Henry Spencer (which was
29  optionally, minimizing in Perl) the amount of the subject that matches  subsequently heavily modified for Perl) compiles the expression twice: once in
30  individual wild portions of the pattern. This is an "NFA algorithm" in Friedl's  a dummy mode in order to find out how much store will be needed, and then for
31  terminology.  real. (The Perl version probably doesn't do this any more; I'm talking about
32    the original library.) The execution function operates by backtracking and
33  For the set of functions that forms PCRE (which are unrelated to those  maximizing (or, optionally, minimizing in Perl) the amount of the subject that
34  mentioned above), I tried at first to invent an algorithm that used an amount  matches individual wild portions of the pattern. This is an "NFA algorithm" in
35  of store bounded by a multiple of the number of characters in the pattern, to  Friedl's terminology.
36  save on compiling time. However, because of the greater complexity in Perl  
37  regular expressions, I couldn't do this. In any case, a first pass through the  OK, here's the real stuff
38  pattern is needed, in order to find internal flag settings like (?i) at top  -------------------------
39  level. So PCRE works by running a very degenerate first pass to calculate a  
40  maximum store size, and then a second pass to do the real compile - which may  For the set of functions that form the "basic" PCRE library (which are
41  use a bit less than the predicted amount of store. The idea is that this is  unrelated to those mentioned above), I tried at first to invent an algorithm
42  going to turn out faster because the first pass is degenerate and the second  that used an amount of store bounded by a multiple of the number of characters
43  pass can just store stuff straight into the vector. It does make the compiling  in the pattern, to save on compiling time. However, because of the greater
44  functions bigger, of course, but they have got quite big anyway to handle all  complexity in Perl regular expressions, I couldn't do this. In any case, a
45  the Perl stuff.  first pass through the pattern is helpful for other reasons.
46    
47    Computing the memory requirement: how it was
48    --------------------------------------------
49    
50    Up to and including release 6.7, PCRE worked by running a very degenerate first
51    pass to calculate a maximum store size, and then a second pass to do the real
52    compile - which might use a bit less than the predicted amount of memory. The
53    idea was that this would turn out faster than the Henry Spencer code because
54    the first pass is degenerate and the second pass can just store stuff straight
55    into the vector, which it knows is big enough.
56    
57    Computing the memory requirement: how it is
58    -------------------------------------------
59    
60    By the time I was working on a potential 6.8 release, the degenerate first pass
61    had become very complicated and hard to maintain. Indeed one of the early
62    things I did for 6.8 was to fix Yet Another Bug in the memory computation. Then
63    I had a flash of inspiration as to how I could run the real compile function in
64    a "fake" mode that enables it to compute how much memory it would need, while
65    actually only ever using a few hundred bytes of working memory, and without too
66    many tests of the mode that might slow it down. So I re-factored the compiling
67    functions to work this way. This got rid of about 600 lines of source. It
68    should make future maintenance and development easier. As this was such a major
69    change, I never released 6.8, instead upping the number to 7.0 (other quite
70    major changes were also present in the 7.0 release).
71    
72    A side effect of this work was that the previous limit of 200 on the nesting
73    depth of parentheses was removed. However, there is a downside: pcre_compile()
74    runs more slowly than before (30% or more, depending on the pattern) because it
75    is doing a full analysis of the pattern. My hope was that this would not be a
76    big issue, and in the event, nobody has commented on it.
77    
78    Traditional matching function
79    -----------------------------
80    
81    The "traditional", and original, matching function is called pcre_exec(), and
82    it implements an NFA algorithm, similar to the original Henry Spencer algorithm
83    and the way that Perl works. This is not surprising, since it is intended to be
84    as compatible with Perl as possible. This is the function most users of PCRE
85    will use most of the time.
86    
87    Supplementary matching function
88    -------------------------------
89    
90    From PCRE 6.0, there is also a supplementary matching function called
91    pcre_dfa_exec(). This implements a DFA matching algorithm that searches
92    simultaneously for all possible matches that start at one point in the subject
93    string. (Going back to my roots: see Historical Note 1 above.) This function
94    intreprets the same compiled pattern data as pcre_exec(); however, not all the
95    facilities are available, and those that are do not always work in quite the
96    same way. See the user documentation for details.
97    
98    The algorithm that is used for pcre_dfa_exec() is not a traditional FSM,
99    because it may have a number of states active at one time. More work would be
100    needed at compile time to produce a traditional FSM where only one state is
101    ever active at once. I believe some other regex matchers work this way.
102    
103    
104    Format of compiled patterns
105    ---------------------------
106    
107  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
108  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
109  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
110  follow it. A list of all the opcodes follows:  follow it.
111    
112    In many cases below LINK_SIZE data values are specified for offsets within the
113    compiled pattern. The default value for LINK_SIZE is 2, but PCRE can be
114    compiled to use 3-byte or 4-byte values for these offsets (impairing the
115    performance). This is necessary only when patterns whose compiled length is
116    greater than 64K are going to be processed. In this description, we assume the
117    "normal" compilation options. Data values that are counts (e.g. for
118    quantifiers) are always just two bytes long.
119    
120    A list of the opcodes follows:
121    
122    
123  Opcodes with no following data  Opcodes with no following data
124  ------------------------------  ------------------------------
# Line 48  Opcodes with no following data Line 126  Opcodes with no following data
126  These items are all just one byte long  These items are all just one byte long
127    
128    OP_END                 end of pattern    OP_END                 end of pattern
129    OP_ANY                 match any character    OP_ANY                 match any one character other than newline
130      OP_ALLANY              match any one character, including newline
131      OP_ANYBYTE             match any single byte, even in UTF-8 mode
132    OP_SOD                 match start of data: \A    OP_SOD                 match start of data: \A
133      OP_SOM,                start of match (subject + offset): \G
134      OP_SET_SOM,            set start of match (\K)
135    OP_CIRC                ^ (start of data, or after \n in multiline)    OP_CIRC                ^ (start of data, or after \n in multiline)
136    OP_NOT_WORD_BOUNDARY   \W    OP_NOT_WORD_BOUNDARY   \W
137    OP_WORD_BOUNDARY       \w    OP_WORD_BOUNDARY       \w
138    OP_NOT_DIGIT           \D    OP_NOT_DIGIT           \D
139    OP_DIGIT               \d    OP_DIGIT               \d
140      OP_NOT_HSPACE          \H
141      OP_HSPACE              \h
142    OP_NOT_WHITESPACE      \S    OP_NOT_WHITESPACE      \S
143    OP_WHITESPACE          \s    OP_WHITESPACE          \s
144      OP_NOT_VSPACE          \V
145      OP_VSPACE              \v
146    OP_NOT_WORDCHAR        \W    OP_NOT_WORDCHAR        \W
147    OP_WORDCHAR            \w    OP_WORDCHAR            \w
148    OP_EODN                match end of data or \n at end: \Z    OP_EODN                match end of data or \n at end: \Z
149    OP_EOD                 match end of data: \z    OP_EOD                 match end of data: \z
150    OP_DOLL                $ (end of data, or before \n in multiline)    OP_DOLL                $ (end of data, or before \n in multiline)
151    OP_RECURSE             match the pattern recursively    OP_EXTUNI              match an extended Unicode character
152      OP_ANYNL               match any Unicode newline sequence
153    
154      OP_ACCEPT              ) These are Perl 5.10's "backtracking
155      OP_COMMIT              ) control verbs". If OP_ACCEPT is inside
156      OP_FAIL                ) capturing parentheses, it may be preceded
157      OP_PRUNE               ) by one or more OP_CLOSE, followed by a 2-byte
158      OP_SKIP                ) number, indicating which parentheses must be
159      OP_THEN                ) closed.
160    
161    
162  Repeating single characters  Repeating single characters
163  ---------------------------  ---------------------------
164    
165  The common repeats (*, +, ?) when applied to a single character appear as  The common repeats (*, +, ?) when applied to a single character use the
166  two-byte items using the following opcodes:  following opcodes:
167    
168    OP_STAR    OP_STAR
169    OP_MINSTAR    OP_MINSTAR
170      OP_POSSTAR
171    OP_PLUS    OP_PLUS
172    OP_MINPLUS    OP_MINPLUS
173      OP_POSPLUS
174    OP_QUERY    OP_QUERY
175    OP_MINQUERY    OP_MINQUERY
176      OP_POSQUERY
177    
178  Those with "MIN" in their name are the minimizing versions. Each is followed by  In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.
179  the character that is to be repeated. Other repeats make use of  Those with "MIN" in their name are the minimizing versions. Those with "POS" in
180    their names are possessive versions. Each is followed by the character that is
181    to be repeated. Other repeats make use of
182    
183    OP_UPTO    OP_UPTO
184    OP_MINUPTO    OP_MINUPTO
185      OP_POSUPTO
186    OP_EXACT    OP_EXACT
187    
188  which are followed by a two-byte count (most significant first) and the  which are followed by a two-byte count (most significant first) and the
189  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
190  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
191  OP_UPTO (or OP_MINUPTO).  OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).
192    
193    
194  Repeating character types  Repeating character types
# Line 100  byte. The opcodes are: Line 200  byte. The opcodes are:
200    
201    OP_TYPESTAR    OP_TYPESTAR
202    OP_TYPEMINSTAR    OP_TYPEMINSTAR
203      OP_TYPEPOSSTAR
204    OP_TYPEPLUS    OP_TYPEPLUS
205    OP_TYPEMINPLUS    OP_TYPEMINPLUS
206      OP_TYPEPOSPLUS
207    OP_TYPEQUERY    OP_TYPEQUERY
208    OP_TYPEMINQUERY    OP_TYPEMINQUERY
209      OP_TYPEPOSQUERY
210    OP_TYPEUPTO    OP_TYPEUPTO
211    OP_TYPEMINUPTO    OP_TYPEMINUPTO
212      OP_TYPEPOSUPTO
213    OP_TYPEEXACT    OP_TYPEEXACT
214    
215    
216  Matching a character string  Match by Unicode property
217    -------------------------
218    
219    OP_PROP and OP_NOTPROP are used for positive and negative matches of a
220    character by testing its Unicode property (the \p and \P escape sequences).
221    Each is followed by two bytes that encode the desired property as a type and a
222    value.
223    
224    Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by
225    three bytes: OP_PROP or OP_NOTPROP and then the desired property type and
226    value.
227    
228    
229    Matching literal characters
230  ---------------------------  ---------------------------
231    
232  The OP_CHARS opcode is followed by a one-byte count and then that number of  The OP_CHAR opcode is followed by a single character that is to be matched
233  characters. If there are more than 255 characters in sequence, successive  casefully. For caseless matching, OP_CHARNC is used. In UTF-8 mode, the
234  instances of OP_CHARS are used.  character may be more than one byte long. (Earlier versions of PCRE used
235    multi-character strings, but this was changed to allow some new features to be
236    added.)
237    
238    
239  Character classes  Character classes
240  -----------------  -----------------
241    
242  OP_CLASS is used for a character class, provided there are at least two  If there is only one character, OP_CHAR or OP_CHARNC is used for a positive
243  characters in the class. If there is only one character, OP_CHARS is used for a  class, and OP_NOT for a negative one (that is, for something like [^a]).
244  positive class, and OP_NOT for a negative one (that is, for something like  However, in UTF-8 mode, the use of OP_NOT applies only to characters with
245  [^a]). Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a  values < 128, because OP_NOT is confined to single bytes.
246  repeated, negated, single-character class. The normal ones (OP_STAR etc.) are  
247  used for a repeated positive single-character class.  Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated,
248    negated, single-character class. The normal ones (OP_STAR etc.) are used for a
249  OP_CLASS is followed by a 32-byte bit map containing a 1 bit for every  repeated positive single-character class.
250  character that is acceptable. The bits are counted from the least significant  
251  end of each byte.  When there's more than one character in a class and all the characters are less
252    than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a negative
253    one. In either case, the opcode is followed by a 32-byte bit map containing a 1
254    bit for every character that is acceptable. The bits are counted from the least
255    significant end of each byte.
256    
257    The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,
258    subject characters with values greater than 256 can be handled correctly. For
259    OP_CLASS they don't match, whereas for OP_NCLASS they do.
260    
261    For classes containing characters with values > 255, OP_XCLASS is used. It
262    optionally uses a bit map (if any characters lie within it), followed by a list
263    of pairs and single characters. There is a flag character than indicates
264    whether it's a positive or a negative class.
265    
266    
267  Back references  Back references
268  ---------------  ---------------
269    
270  OP_REF is followed by a single byte containing the reference number.  OP_REF is followed by two bytes containing the reference number.
271    
272    
273  Repeating character classes and back references  Repeating character classes and back references
274  -----------------------------------------------  -----------------------------------------------
275    
276  Single-character classes are handled specially (see above). This applies to  Single-character classes are handled specially (see above). This section
277  OP_CLASS and OP_REF. In both cases, the repeat information follows the base  applies to OP_CLASS and OP_REF. In both cases, the repeat information follows
278  item. The matching code looks at the following opcode to see if it is one of  the base item. The matching code looks at the following opcode to see if it is
279    one of
280    
281    OP_CRSTAR    OP_CRSTAR
282    OP_CRMINSTAR    OP_CRMINSTAR
# Line 155  item. The matching code looks at the fol Line 288  item. The matching code looks at the fol
288    OP_CRMINRANGE    OP_CRMINRANGE
289    
290  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
291  four bytes of data, comprising the minimum and maximum repeat counts.  four bytes of data, comprising the minimum and maximum repeat counts. There are
292    no special possessive opcodes for these repeats; a possessive repeat is
293    compiled into an atomic group.
294    
295    
296  Brackets and alternation  Brackets and alternation
# Line 163  Brackets and alternation Line 298  Brackets and alternation
298    
299  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
300  compile time, so alternation always happens in the context of brackets.  compile time, so alternation always happens in the context of brackets.
301  Non-capturing brackets use the opcode OP_BRA, while capturing brackets use  
302  OP_BRA+1, OP_BRA+2, etc. [Note for North Americans: "bracket" to some English  [Note for North Americans: "bracket" to some English speakers, including
303  speakers, including myself, can be round, square, curly, or pointy. Hence this  myself, can be round, square, curly, or pointy. Hence this usage.]
304  usage.]  
305    Non-capturing brackets use the opcode OP_BRA. Originally PCRE was limited to 99
306  A bracket opcode is followed by two bytes which give the offset to the next  capturing brackets and it used a different opcode for each one. From release
307  alternative OP_ALT or, if there aren't any branches, to the matching KET  3.5, the limit was removed by putting the bracket number into the data for
308  opcode. Each OP_ALT is followed by two bytes giving the offset to the next one,  higher-numbered brackets. From release 7.0 all capturing brackets are handled
309  or to the KET opcode.  this way, using the single opcode OP_CBRA.
310    
311    A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
312    next alternative OP_ALT or, if there aren't any branches, to the matching
313    OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
314    the next one, or to the OP_KET opcode. For capturing brackets, the bracket
315    number immediately follows the offset, always as a 2-byte item.
316    
317  OP_KET is used for subpatterns that do not repeat indefinitely, while  OP_KET is used for subpatterns that do not repeat indefinitely, while
318  OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or  OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
319  maximally respectively. All three are followed by two bytes giving (as a  maximally respectively. All three are followed by LINK_SIZE bytes giving (as a
320  positive number) the offset back to the matching BRA opcode.  positive number) the offset back to the matching bracket opcode.
321    
322  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
323  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
324  opcodes which tell the matcher that skipping this subpattern entirely is a  single-byte opcodes that tell the matcher that skipping the following
325  valid branch.  subpattern entirely is a valid branch. In the case of the first two, not
326    skipping the pattern is also valid (greedy and non-greedy). The third is used
327    when a pattern has the quantifier {0,0}. It cannot be entirely discarded,
328    because it may be called as a subroutine from elsewhere in the regex.
329    
330  A subpattern with an indefinite maximum repetition is replicated in the  A subpattern with an indefinite maximum repetition is replicated in the
331  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
332  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
333  appropriate.  as appropriate.
334    
335  A subpattern with a bounded maximum repetition is replicated in a nested  A subpattern with a bounded maximum repetition is replicated in a nested
336  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
337  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
338  compiled as (abc)(abc)((abc)((abc)(abc)?)?)?. The 200-bracket limit does not  compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group
339  apply to these internally generated brackets.  has the same number.
340    
341    When a repeated subpattern has an unbounded upper limit, it is checked to see
342    whether it could match an empty string. If this is the case, the opcode in the
343    final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher
344    that it needs to check for matching an empty string when it hits OP_KETRMIN or
345    OP_KETRMAX, and if so, to break the loop.
346    
347    
348  Assertions  Assertions
# Line 208  each alternative of a lookbehind asserti Line 358  each alternative of a lookbehind asserti
358  fixed lengths.  fixed lengths.
359    
360    
361  Once-only subpatterns  Once-only (atomic) subpatterns
362  ---------------------  ------------------------------
363    
364  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
365  OP_ONCE.  OP_ONCE. The check for matching an empty string in an unbounded repeat is
366    handled entirely at runtime, so there is just this one opcode.
367    
368    
369  Conditional subpatterns  Conditional subpatterns
370  -----------------------  -----------------------
371    
372  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
373    OP_SCOND for one that might match an empty string in an unbounded repeat. If
374  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
375  subpattern using the opcode OP_CREF followed by one byte containing the  subpattern using the opcode OP_CREF followed by two bytes containing the
376  reference number. Otherwise, a conditional subpattern will always start with  reference number. OP_NCREF is used instead if the reference was generated by
377  one of the assertions.  name (so that the runtime code knows to check for duplicate names).
378    
379    If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
380    group x" (coded as "(?(Rx)"), the group number is stored at the start of the
381    subpattern using the opcode OP_RREF or OP_NRREF (cf OP_NCREF), and a value of
382    zero for "the whole pattern". For a DEFINE condition, just the single byte
383    OP_DEF is used (it has no associated data). Otherwise, a conditional subpattern
384    always starts with one of the assertions.
385    
386    
387    Recursion
388    ---------
389    
390    Recursion either matches the current regex, or some subexpression. The opcode
391    OP_RECURSE is followed by an value which is the offset to the starting bracket
392    from the start of the whole pattern. From release 6.5, OP_RECURSE is
393    automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
394    broke it). OP_RECURSE is also used for "subroutine" calls, even though they
395    are not strictly a recursion.
396    
397    
398    Callout
399    -------
400    
401    OP_CALLOUT is followed by one byte of data that holds a callout number in the
402    range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
403    cases there follows a two-byte value giving the offset in the pattern to the
404    start of the following item, and another two-byte item giving the length of the
405    next item.
406    
407    
408  Changing options  Changing options
409  ----------------  ----------------
410    
411  If any of the /i, /m, or /s options are changed within a parenthesized group,  If any of the /i, /m, or /s options are changed within a pattern, an OP_OPT
412  an OP_OPT opcode is compiled, followed by one byte containing the new settings  opcode is compiled, followed by one byte containing the new settings of these
413  of these flags. If there are several alternatives in a group, there is an  flags. If there are several alternatives, there is an occurrence of OP_OPT at
414  occurrence of OP_OPT at the start of all those following the first options  the start of all those following the first options change, to set appropriate
415  change, to set appropriate options for the start of the alternative.  options for the start of the alternative. Immediately after the end of the
416  Immediately after the end of the group there is another such item to reset the  group there is another such item to reset the flags to their previous values. A
417  flags to their previous values. Other changes of flag within the pattern can be  change of flag right at the very start of the pattern can be handled entirely
418  handled entirely at compile time, and so do not cause anything to be put into  at compile time, and so does not cause anything to be put into the compiled
419  the compiled data.  data.
   
420    
421  Philip Hazel  Philip Hazel
422  August 2000  October 2009
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