trunk/doc/Tech.Notes

Technical Notes about PCRE
--------------------------

Historical note 1
-----------------

Many years ago I implemented some regular expression functions to an algorithm
suggested by Martin Richards. These were not Unix-like in form, and were quite
restricted in what they could do by comparison with Perl. The interesting part
about the algorithm was that the amount of space required to hold the compiled
form of an expression was known in advance. The code to apply an expression did
not operate by backtracking, as the original Henry Spencer code and current
Perl code does, but instead checked all possibilities simultaneously by keeping
a list of current states and checking all of them as it advanced through the
subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA
algorithm". When the pattern was all used up, all remaining states were
possible matches, and the one matching the longest subset of the subject string
was chosen. This did not necessarily maximize the individual wild portions of
the pattern, as is expected in Unix and Perl-style regular expressions.

Historical note 2
-----------------

By contrast, the code originally written by Henry Spencer and subsequently
heavily modified for Perl actually compiles the expression twice: once in a
dummy mode in order to find out how much store will be needed, and then for
real. The execution function operates by backtracking and maximizing (or,
optionally, minimizing in Perl) the amount of the subject that matches
individual wild portions of the pattern. This is an "NFA algorithm" in Friedl's
terminology.

OK, here's the real stuff
-------------------------

For the set of functions that form the "basic" PCRE library (which are
unrelated to those mentioned above), I tried at first to invent an algorithm
that used an amount of store bounded by a multiple of the number of characters
in the pattern, to save on compiling time. However, because of the greater
complexity in Perl regular expressions, I couldn't do this. In any case, a
first pass through the pattern is needed, for a number of reasons. PCRE works
by running a very degenerate first pass to calculate a maximum store size, and
then a second pass to do the real compile - which may use a bit less than the
predicted amount of store. The idea is that this is going to turn out faster
because the first pass is degenerate and the second pass can just store stuff
straight into the vector, which it knows is big enough. It does make the
compiling functions bigger, of course, but they have got quite big anyway to
handle all the Perl stuff.

Traditional matching function
-----------------------------

The "traditional", and original, matching function is called pcre_exec(), and 
it implements an NFA algorithm, similar to the original Henry Spencer algorithm 
and the way that Perl works. Not surprising, since it is intended to be as 
compatible with Perl as possible. This is the function most users of PCRE will 
use most of the time.

Supplementary matching function
-------------------------------

From PCRE 6.0, there is also a supplementary matching function called 
pcre_dfa_exec(). This implements a DFA matching algorithm that searches 
simultaneously for all possible matches that start at one point in the subject 
string. (Going back to my roots: see Historical Note 1 above.) This function 
intreprets the same compiled pattern data as pcre_exec(); however, not all the 
facilities are available, and those that are don't always work in quite the 
same way. See the user documentation for details.

Format of compiled patterns
---------------------------

The compiled form of a pattern is a vector of bytes, containing items of
variable length. The first byte in an item is an opcode, and the length of the
item is either implicit in the opcode or contained in the data bytes that
follow it. 

In many cases below "two-byte" data values are specified. This is in fact just
a default. PCRE can be compiled to use 3-byte or 4-byte values (impairing the
performance). This is necessary only when patterns whose compiled length is
greater than 64K are going to be processed. In this description, we assume the 
"normal" compilation options.

A list of all the opcodes follows:

Opcodes with no following data
------------------------------

These items are all just one byte long

  OP_END                 end of pattern
  OP_ANY                 match any character
  OP_ANYBYTE             match any single byte, even in UTF-8 mode
  OP_SOD                 match start of data: \A
  OP_SOM,                start of match (subject + offset): \G
  OP_CIRC                ^ (start of data, or after \n in multiline)
  OP_NOT_WORD_BOUNDARY   \W
  OP_WORD_BOUNDARY       \w
  OP_NOT_DIGIT           \D
  OP_DIGIT               \d
  OP_NOT_WHITESPACE      \S
  OP_WHITESPACE          \s
  OP_NOT_WORDCHAR        \W
  OP_WORDCHAR            \w
  OP_EODN                match end of data or \n at end: \Z
  OP_EOD                 match end of data: \z
  OP_DOLL                $ (end of data, or before \n in multiline)
  OP_EXTUNI              match an extended Unicode character 
  

Repeating single characters
---------------------------

The common repeats (*, +, ?) when applied to a single character use the
following opcodes:

  OP_STAR
  OP_MINSTAR
  OP_PLUS
  OP_MINPLUS
  OP_QUERY
  OP_MINQUERY

In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.
Those with "MIN" in their name are the minimizing versions. Each is followed by
the character that is to be repeated. Other repeats make use of

  OP_UPTO
  OP_MINUPTO
  OP_EXACT

which are followed by a two-byte count (most significant first) and the
repeated character. OP_UPTO matches from 0 to the given number. A repeat with a
non-zero minimum and a fixed maximum is coded as an OP_EXACT followed by an
OP_UPTO (or OP_MINUPTO).


Repeating character types
-------------------------

Repeats of things like \d are done exactly as for single characters, except
that instead of a character, the opcode for the type is stored in the data
byte. The opcodes are:

  OP_TYPESTAR
  OP_TYPEMINSTAR
  OP_TYPEPLUS
  OP_TYPEMINPLUS
  OP_TYPEQUERY
  OP_TYPEMINQUERY
  OP_TYPEUPTO
  OP_TYPEMINUPTO
  OP_TYPEEXACT


Match by Unicode property
-------------------------

OP_PROP and OP_NOTPROP are used for positive and negative matches of a 
character by testing its Unicode property (the \p and \P escape sequences).
Each is followed by a single byte that encodes the desired property value.

Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by two 
bytes: OP_PROP or OP_NOTPROP and then the desired property 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.)


Character classes
-----------------

If there is only one character, OP_CHAR or OP_CHARNC is used for a positive
class, and OP_NOT for a negative one (that is, for something like [^a]).
However, in UTF-8 mode, the use of OP_NOT applies only to characters with
values < 128, because OP_NOT is confined to single bytes.

Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated,
negated, single-character class. The normal ones (OP_STAR etc.) are used for a
repeated positive single-character class.

When there's more than one character in a class and all the characters are less
than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a negative
one. In either case, the opcode is followed by a 32-byte bit map containing a 1
bit for every character that is acceptable. The bits are counted from the least
significant end of each byte.

The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,
subject characters with values greater than 256 can be handled correctly. For
OP_CLASS they don't match, whereas for OP_NCLASS they do.

For classes containing characters with values > 255, OP_XCLASS is used. It
optionally uses a bit map (if any characters lie within it), followed by a list
of pairs and single characters. There is a flag character than indicates
whether it's a positive or a negative class.


Back references
---------------

OP_REF is followed by two bytes containing the reference number.


Repeating character classes and back references
-----------------------------------------------

Single-character classes are handled specially (see above). This applies to
OP_CLASS and OP_REF. In both cases, the repeat information follows the base
item. The matching code looks at the following opcode to see if it is one of

  OP_CRSTAR
  OP_CRMINSTAR
  OP_CRPLUS
  OP_CRMINPLUS
  OP_CRQUERY
  OP_CRMINQUERY
  OP_CRRANGE
  OP_CRMINRANGE

All but the last two are just single-byte items. The others are followed by
four bytes of data, comprising the minimum and maximum repeat counts.


Brackets and alternation
------------------------

A pair of non-capturing (round) brackets is wrapped round each expression at
compile time, so alternation always happens in the context of brackets.

Non-capturing brackets use the opcode OP_BRA, while capturing brackets use
OP_BRA+1, OP_BRA+2, etc. [Note for North Americans: "bracket" to some English
speakers, including myself, can be round, square, curly, or pointy. Hence this
usage.]

Originally PCRE was limited to 99 capturing brackets (so as not to use up all
the opcodes). From release 3.5, there is no limit. What happens is that the
first ones, up to EXTRACT_BASIC_MAX are handled with separate opcodes, as
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.)

A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
next alternative OP_ALT or, if there aren't any branches, to the matching
OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
the next one, or to the OP_KET opcode.

OP_KET is used for subpatterns that do not repeat indefinitely, while
OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
maximally respectively. All three are followed by LINK_SIZE bytes giving (as a
positive number) the offset back to the matching OP_BRA opcode.

If a subpattern is quantified such that it is permitted to match zero times, it
is preceded by one of OP_BRAZERO or OP_BRAMINZERO. These are single-byte
opcodes which tell the matcher that skipping this subpattern entirely is a
valid branch.

A subpattern with an indefinite maximum repetition is replicated in the
compiled data its minimum number of times (or once with OP_BRAZERO if the
minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
as appropriate.

A subpattern with a bounded maximum repetition is replicated in a nested
fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
before each replication after the minimum, so that, for example, (abc){2,5} is
compiled as (abc)(abc)((abc)((abc)(abc)?)?)?.


Assertions
----------

Forward assertions are just like other subpatterns, but starting with one of
the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
is OP_REVERSE, followed by a two byte count of the number of characters to move
back the pointer in the subject string. When operating in UTF-8 mode, the count
is a character count rather than a byte count. A separate count is present in
each alternative of a lookbehind assertion, allowing them to have different
fixed lengths.


Once-only subpatterns
---------------------

These are also just like other subpatterns, but they start with the opcode
OP_ONCE.


Conditional subpatterns
-----------------------

These are like other subpatterns, but they start with the opcode OP_COND. If
the condition is a back reference, this is stored at the start of the
subpattern using the opcode OP_CREF followed by two bytes containing the
reference number. If the condition is "in recursion" (coded as "(?(R)"), the
same scheme is used, with a "reference number" of 0xffff. Otherwise, a
conditional subpattern always starts with one of the assertions.


Recursion
---------

Recursion either matches the current regex, or some subexpression. The opcode
OP_RECURSE is followed by an value which is the offset to the starting bracket
from the start of the whole pattern. From release 6.5, OP_RECURSE is 
automatically wrapped inside OP_ONCE brackets (because otherwise some patterns 
broke it). OP_RECURSE is also used for "subroutine" calls, even though they 
are not strictly a recursion.


Callout
-------

OP_CALLOUT is followed by one byte of data that holds a callout number in the
range 0 to 254 for manual callouts, or 255 for an automatic callout. In both 
cases there follows a two-byte value giving the offset in the pattern to the
start of the following item, and another two-byte item giving the length of the
next item.


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.

Philip Hazel
January 2006
1	nigel	41	Technical Notes about PCRE
2			--------------------------
3
4	nigel	75	Historical note 1
5			-----------------
6
7	nigel	41	Many years ago I implemented some regular expression functions to an algorithm
8			suggested by Martin Richards. These were not Unix-like in form, and were quite
9			restricted in what they could do by comparison with Perl. The interesting part
10			about the algorithm was that the amount of space required to hold the compiled
11			form of an expression was known in advance. The code to apply an expression did
12	nigel	63	not operate by backtracking, as the original Henry Spencer code and current
13			Perl code does, but instead checked all possibilities simultaneously by keeping
14			a list of current states and checking all of them as it advanced through the
15	nigel	75	subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA
16			algorithm". When the pattern was all used up, all remaining states were
17	nigel	63	possible matches, and the one matching the longest subset of the subject string
18			was chosen. This did not necessarily maximize the individual wild portions of
19			the pattern, as is expected in Unix and Perl-style regular expressions.
20	nigel	41
21	nigel	75	Historical note 2
22			-----------------
23
24	nigel	41	By contrast, the code originally written by Henry Spencer and subsequently
25			heavily modified for Perl actually compiles the expression twice: once in a
26			dummy mode in order to find out how much store will be needed, and then for
27			real. The execution function operates by backtracking and maximizing (or,
28			optionally, minimizing in Perl) the amount of the subject that matches
29			individual wild portions of the pattern. This is an "NFA algorithm" in Friedl's
30			terminology.
31
32	nigel	75	OK, here's the real stuff
33			-------------------------
34
35	nigel	77	For the set of functions that form the "basic" PCRE library (which are
36			unrelated to those mentioned above), I tried at first to invent an algorithm
37			that used an amount of store bounded by a multiple of the number of characters
38			in the pattern, to save on compiling time. However, because of the greater
39			complexity in Perl regular expressions, I couldn't do this. In any case, a
40			first pass through the pattern is needed, for a number of reasons. PCRE works
41			by running a very degenerate first pass to calculate a maximum store size, and
42			then a second pass to do the real compile - which may use a bit less than the
43			predicted amount of store. The idea is that this is going to turn out faster
44			because the first pass is degenerate and the second pass can just store stuff
45			straight into the vector, which it knows is big enough. It does make the
46			compiling functions bigger, of course, but they have got quite big anyway to
47			handle all the Perl stuff.
48	nigel	41
49	nigel	77	Traditional matching function
50			-----------------------------
51
52			The "traditional", and original, matching function is called pcre_exec(), and
53			it implements an NFA algorithm, similar to the original Henry Spencer algorithm
54			and the way that Perl works. Not surprising, since it is intended to be as
55			compatible with Perl as possible. This is the function most users of PCRE will
56			use most of the time.
57
58			Supplementary matching function
59			-------------------------------
60
61			From PCRE 6.0, there is also a supplementary matching function called
62			pcre_dfa_exec(). This implements a DFA matching algorithm that searches
63			simultaneously for all possible matches that start at one point in the subject
64			string. (Going back to my roots: see Historical Note 1 above.) This function
65			intreprets the same compiled pattern data as pcre_exec(); however, not all the
66			facilities are available, and those that are don't always work in quite the
67			same way. See the user documentation for details.
68
69			Format of compiled patterns
70			---------------------------
71
72	nigel	41	The compiled form of a pattern is a vector of bytes, containing items of
73			variable length. The first byte in an item is an opcode, and the length of the
74	nigel	75	item is either implicit in the opcode or contained in the data bytes that
75			follow it.
76	nigel	41
77	nigel	75	In many cases below "two-byte" data values are specified. This is in fact just
78			a default. PCRE can be compiled to use 3-byte or 4-byte values (impairing the
79			performance). This is necessary only when patterns whose compiled length is
80			greater than 64K are going to be processed. In this description, we assume the
81			"normal" compilation options.
82
83			A list of all the opcodes follows:
84
85	nigel	41	Opcodes with no following data
86			------------------------------
87
88			These items are all just one byte long
89
90			OP_END end of pattern
91			OP_ANY match any character
92	nigel	75	OP_ANYBYTE match any single byte, even in UTF-8 mode
93	nigel	41	OP_SOD match start of data: \A
94	nigel	71	OP_SOM, start of match (subject + offset): \G
95	nigel	41	OP_CIRC ^ (start of data, or after \n in multiline)
96			OP_NOT_WORD_BOUNDARY \W
97			OP_WORD_BOUNDARY \w
98			OP_NOT_DIGIT \D
99			OP_DIGIT \d
100			OP_NOT_WHITESPACE \S
101			OP_WHITESPACE \s
102			OP_NOT_WORDCHAR \W
103			OP_WORDCHAR \w
104			OP_EODN match end of data or \n at end: \Z
105			OP_EOD match end of data: \z
106			OP_DOLL $ (end of data, or before \n in multiline)
107	nigel	75	OP_EXTUNI match an extended Unicode character
108
109	nigel	41
110			Repeating single characters
111			---------------------------
112
113	nigel	75	The common repeats (*, +, ?) when applied to a single character use the
114			following opcodes:
115	nigel	41
116			OP_STAR
117			OP_MINSTAR
118			OP_PLUS
119			OP_MINPLUS
120			OP_QUERY
121			OP_MINQUERY
122
123	nigel	75	In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.
124	nigel	41	Those with "MIN" in their name are the minimizing versions. Each is followed by
125			the character that is to be repeated. Other repeats make use of
126
127			OP_UPTO
128			OP_MINUPTO
129			OP_EXACT
130
131			which are followed by a two-byte count (most significant first) and the
132			repeated character. OP_UPTO matches from 0 to the given number. A repeat with a
133			non-zero minimum and a fixed maximum is coded as an OP_EXACT followed by an
134			OP_UPTO (or OP_MINUPTO).
135
136
137			Repeating character types
138			-------------------------
139
140			Repeats of things like \d are done exactly as for single characters, except
141			that instead of a character, the opcode for the type is stored in the data
142			byte. The opcodes are:
143
144			OP_TYPESTAR
145			OP_TYPEMINSTAR
146			OP_TYPEPLUS
147			OP_TYPEMINPLUS
148			OP_TYPEQUERY
149			OP_TYPEMINQUERY
150			OP_TYPEUPTO
151			OP_TYPEMINUPTO
152			OP_TYPEEXACT
153
154
155	nigel	75	Match by Unicode property
156			-------------------------
157
158			OP_PROP and OP_NOTPROP are used for positive and negative matches of a
159			character by testing its Unicode property (the \p and \P escape sequences).
160			Each is followed by a single byte that encodes the desired property value.
161
162			Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by two
163			bytes: OP_PROP or OP_NOTPROP and then the desired property value.
164
165
166			Matching literal characters
167	nigel	41	---------------------------
168
169	nigel	75	The OP_CHAR opcode is followed by a single character that is to be matched
170			casefully. For caseless matching, OP_CHARNC is used. In UTF-8 mode, the
171			character may be more than one byte long. (Earlier versions of PCRE used
172			multi-character strings, but this was changed to allow some new features to be
173			added.)
174	nigel	41
175
176			Character classes
177			-----------------
178
179	nigel	75	If there is only one character, OP_CHAR or OP_CHARNC is used for a positive
180			class, and OP_NOT for a negative one (that is, for something like [^a]).
181			However, in UTF-8 mode, the use of OP_NOT applies only to characters with
182			values < 128, because OP_NOT is confined to single bytes.
183	nigel	41
184	nigel	63	Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated,
185			negated, single-character class. The normal ones (OP_STAR etc.) are used for a
186			repeated positive single-character class.
187
188	nigel	71	When there's more than one character in a class and all the characters are less
189	nigel	75	than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a negative
190	nigel	71	one. In either case, the opcode is followed by a 32-byte bit map containing a 1
191			bit for every character that is acceptable. The bits are counted from the least
192			significant end of each byte.
193	nigel	41
194	nigel	75	The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,
195			subject characters with values greater than 256 can be handled correctly. For
196	nigel	71	OP_CLASS they don't match, whereas for OP_NCLASS they do.
197
198	nigel	63	For classes containing characters with values > 255, OP_XCLASS is used. It
199			optionally uses a bit map (if any characters lie within it), followed by a list
200	nigel	75	of pairs and single characters. There is a flag character than indicates
201	nigel	63	whether it's a positive or a negative class.
202	nigel	41
203	nigel	63
204	nigel	41	Back references
205			---------------
206
207	nigel	53	OP_REF is followed by two bytes containing the reference number.
208	nigel	41
209
210			Repeating character classes and back references
211			-----------------------------------------------
212
213			Single-character classes are handled specially (see above). This applies to
214			OP_CLASS and OP_REF. In both cases, the repeat information follows the base
215			item. The matching code looks at the following opcode to see if it is one of
216
217			OP_CRSTAR
218			OP_CRMINSTAR
219			OP_CRPLUS
220			OP_CRMINPLUS
221			OP_CRQUERY
222			OP_CRMINQUERY
223			OP_CRRANGE
224			OP_CRMINRANGE
225
226			All but the last two are just single-byte items. The others are followed by
227			four bytes of data, comprising the minimum and maximum repeat counts.
228
229
230			Brackets and alternation
231			------------------------
232
233	nigel	43	A pair of non-capturing (round) brackets is wrapped round each expression at
234	nigel	41	compile time, so alternation always happens in the context of brackets.
235	nigel	53
236	nigel	43	Non-capturing brackets use the opcode OP_BRA, while capturing brackets use
237	nigel	41	OP_BRA+1, OP_BRA+2, etc. [Note for North Americans: "bracket" to some English
238	nigel	43	speakers, including myself, can be round, square, curly, or pointy. Hence this
239			usage.]
240	nigel	41
241	nigel	53	Originally PCRE was limited to 99 capturing brackets (so as not to use up all
242			the opcodes). From release 3.5, there is no limit. What happens is that the
243			first ones, up to EXTRACT_BASIC_MAX are handled with separate opcodes, as
244			above. If there are more, the opcode is set to EXTRACT_BASIC_MAX+1, and the
245			first operation in the bracket is OP_BRANUMBER, followed by a 2-byte bracket
246			number. This opcode is ignored while matching, but is fished out when handling
247			the bracket itself. (They could have all been done like this, but I was making
248			minimal changes.)
249
250	nigel	77	A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
251			next alternative OP_ALT or, if there aren't any branches, to the matching
252			OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
253			the next one, or to the OP_KET opcode.
254	nigel	41
255			OP_KET is used for subpatterns that do not repeat indefinitely, while
256			OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
257	nigel	77	maximally respectively. All three are followed by LINK_SIZE bytes giving (as a
258	nigel	75	positive number) the offset back to the matching OP_BRA opcode.
259	nigel	41
260			If a subpattern is quantified such that it is permitted to match zero times, it
261			is preceded by one of OP_BRAZERO or OP_BRAMINZERO. These are single-byte
262			opcodes which tell the matcher that skipping this subpattern entirely is a
263			valid branch.
264
265			A subpattern with an indefinite maximum repetition is replicated in the
266	nigel	75	compiled data its minimum number of times (or once with OP_BRAZERO if the
267			minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
268			as appropriate.
269	nigel	41
270			A subpattern with a bounded maximum repetition is replicated in a nested
271	nigel	75	fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
272			before each replication after the minimum, so that, for example, (abc){2,5} is
273			compiled as (abc)(abc)((abc)((abc)(abc)?)?)?.
274	nigel	41
275
276			Assertions
277			----------
278
279			Forward assertions are just like other subpatterns, but starting with one of
280			the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
281			OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
282			is OP_REVERSE, followed by a two byte count of the number of characters to move
283	nigel	49	back the pointer in the subject string. When operating in UTF-8 mode, the count
284			is a character count rather than a byte count. A separate count is present in
285			each alternative of a lookbehind assertion, allowing them to have different
286			fixed lengths.
287	nigel	41
288
289			Once-only subpatterns
290			---------------------
291
292			These are also just like other subpatterns, but they start with the opcode
293			OP_ONCE.
294
295
296			Conditional subpatterns
297			-----------------------
298
299			These are like other subpatterns, but they start with the opcode OP_COND. If
300			the condition is a back reference, this is stored at the start of the
301	nigel	53	subpattern using the opcode OP_CREF followed by two bytes containing the
302	nigel	63	reference number. If the condition is "in recursion" (coded as "(?(R)"), the
303			same scheme is used, with a "reference number" of 0xffff. Otherwise, a
304			conditional subpattern always starts with one of the assertions.
305	nigel	41
306
307	nigel	71	Recursion
308			---------
309
310			Recursion either matches the current regex, or some subexpression. The opcode
311			OP_RECURSE is followed by an value which is the offset to the starting bracket
312	nigel	87	from the start of the whole pattern. From release 6.5, OP_RECURSE is
313			automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
314			broke it). OP_RECURSE is also used for "subroutine" calls, even though they
315			are not strictly a recursion.
316	nigel	71
317
318			Callout
319			-------
320
321	nigel	75	OP_CALLOUT is followed by one byte of data that holds a callout number in the
322			range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
323			cases there follows a two-byte value giving the offset in the pattern to the
324			start of the following item, and another two-byte item giving the length of the
325			next item.
326	nigel	71
327
328	nigel	41	Changing options
329			----------------
330
331	nigel	63	If any of the /i, /m, or /s options are changed within a pattern, an OP_OPT
332			opcode is compiled, followed by one byte containing the new settings of these
333			flags. If there are several alternatives, there is an occurrence of OP_OPT at
334			the start of all those following the first options change, to set appropriate
335			options for the start of the alternative. Immediately after the end of the
336			group there is another such item to reset the flags to their previous values. A
337			change of flag right at the very start of the pattern can be handled entirely
338			at compile time, and so does not cause anything to be put into the compiled
339			data.
340	nigel	41
341			Philip Hazel
342	nigel	87	January 2006