Contents of /code/trunk/HACKING

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

These are very rough technical notes that record potentially useful information 
about PCRE internals.


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", though it was not a traditional Finite State Machine (FSM). 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 (which was
subsequently heavily modified for Perl) 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 Perl version probably doesn't do this any more; I'm talking about
the original library.) 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 helpful for other reasons. 


Computing the memory requirement: how it was
--------------------------------------------

Up to and including release 6.7, PCRE worked by running a very degenerate first
pass to calculate a maximum store size, and then a second pass to do the real
compile - which might use a bit less than the predicted amount of memory. The
idea was that this would turn out faster than the Henry Spencer code because
the first pass is degenerate and the second pass can just store stuff straight
into the vector, which it knows is big enough.


Computing the memory requirement: how it is
-------------------------------------------

By the time I was working on a potential 6.8 release, the degenerate first pass
had become very complicated and hard to maintain. Indeed one of the early
things I did for 6.8 was to fix Yet Another Bug in the memory computation. Then
I had a flash of inspiration as to how I could run the real compile function in
a "fake" mode that enables it to compute how much memory it would need, while
actually only ever using a few hundred bytes of working memory, and without too
many tests of the mode that might slow it down. So I re-factored the compiling
functions to work this way. This got rid of about 600 lines of source. It
should make future maintenance and development easier. As this was such a major 
change, I never released 6.8, instead upping the number to 7.0 (other quite 
major changes were also present in the 7.0 release).

A side effect of this work was that the previous limit of 200 on the nesting
depth of parentheses was removed. However, there is a downside: pcre_compile()
runs more slowly than before (30% or more, depending on the pattern) because it
is doing a full analysis of the pattern. My hope was that this would not be a
big issue, and in the event, nobody has commented on it.


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. This is 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 do not always work in quite the 
same way. See the user documentation for details.

The algorithm that is used for pcre_dfa_exec() is not a traditional FSM, 
because it may have a number of states active at one time. More work would be 
needed at compile time to produce a traditional FSM where only one state is 
ever active at once. I believe some other regex matchers work this way.


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 LINK_SIZE data values are specified for offsets within the 
compiled pattern. The default value for LINK_SIZE is 2, but PCRE can be
compiled to use 3-byte or 4-byte values for these offsets (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. Data values that are counts (e.g. for
quantifiers) are always just two bytes long.

A list of 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 one character other than newline
  OP_ALLANY              match any one character, including newline
  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_SET_SOM,            set start of match (\K) 
  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_HSPACE          \H
  OP_HSPACE              \h  
  OP_NOT_WHITESPACE      \S
  OP_WHITESPACE          \s
  OP_NOT_VSPACE          \V
  OP_VSPACE              \v  
  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 
  OP_ANYNL               match any Unicode newline sequence 
  
  OP_ACCEPT              ) These are Perl 5.10's "backtracking control   
  OP_COMMIT              ) verbs". If OP_ACCEPT is inside capturing
  OP_FAIL                ) parentheses, it may be preceded by one or more
  OP_PRUNE               ) OP_CLOSE, followed by a 2-byte number,
  OP_SKIP                ) indicating which parentheses must be closed.
  

Backtracking control verbs with data
------------------------------------
 
OP_THEN is followed by a LINK_SIZE offset, which is the distance back to the
start of the current branch.

OP_MARK is followed by the mark name, preceded by a one-byte length, and 
followed by a binary zero. For (*PRUNE), (*SKIP), and (*THEN) with arguments, 
the opcodes OP_PRUNE_ARG, OP_SKIP_ARG, and OP_THEN_ARG are used. For the first 
two, the name follows immediately; for OP_THEN_ARG, it follows the LINK_SIZE 
offset value.
  

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

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

  OP_STAR
  OP_MINSTAR
  OP_POSSTAR 
  OP_PLUS
  OP_MINPLUS
  OP_POSPLUS 
  OP_QUERY
  OP_MINQUERY
  OP_POSQUERY 

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. Those with "POS" in 
their names are possessive versions. Each is followed by the character that is
to be repeated. Other repeats make use of

  OP_UPTO
  OP_MINUPTO
  OP_POSUPTO 
  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 or OPT_POSUPTO).


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_TYPEPOSSTAR 
  OP_TYPEPLUS
  OP_TYPEMINPLUS
  OP_TYPEPOSPLUS 
  OP_TYPEQUERY
  OP_TYPEMINQUERY
  OP_TYPEPOSQUERY 
  OP_TYPEUPTO
  OP_TYPEMINUPTO
  OP_TYPEPOSUPTO 
  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 two bytes that encode the desired property as a type and a 
value.

Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by 
three bytes: OP_PROP or OP_NOTPROP and then the desired property type and 
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 section
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. There are 
no special possessive opcodes for these repeats; a possessive repeat is 
compiled into an atomic group.


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.

[Note for North Americans: "bracket" to some English speakers, including
myself, can be round, square, curly, or pointy. Hence this usage.]

Non-capturing brackets use the opcode OP_BRA. Originally PCRE was limited to 99
capturing brackets and it used a different opcode for each one. From release
3.5, the limit was removed by putting the bracket number into the data for
higher-numbered brackets. From release 7.0 all capturing brackets are handled
this way, using the single opcode OP_CBRA.

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. For capturing brackets, the bracket 
number immediately follows the offset, always as a 2-byte item.

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 bracket opcode.

If a subpattern is quantified such that it is permitted to match zero times, it
is preceded by one of OP_BRAZERO, OP_BRAMINZERO, or OP_SKIPZERO. These are
single-byte opcodes that tell the matcher that skipping the following
subpattern entirely is a valid branch. In the case of the first two, not 
skipping the pattern is also valid (greedy and non-greedy). The third is used 
when a pattern has the quantifier {0,0}. It cannot be entirely discarded, 
because it may be called as a subroutine from elsewhere in the regex.

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)?)?)?, except that each bracketed group 
has the same number.

When a repeated subpattern has an unbounded upper limit, it is checked to see 
whether it could match an empty string. If this is the case, the opcode in the 
final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher
that it needs to check for matching an empty string when it hits OP_KETRMIN or
OP_KETRMAX, and if so, to break the loop.


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 (atomic) subpatterns
------------------------------

These are also just like other subpatterns, but they start with the opcode
OP_ONCE. The check for matching an empty string in an unbounded repeat is 
handled entirely at runtime, so there is just this one opcode.


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

These are like other subpatterns, but they start with the opcode OP_COND, or
OP_SCOND for one that might match an empty string in an unbounded repeat. 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. OP_NCREF is used instead if the reference was generated by 
name (so that the runtime code knows to check for duplicate names).

If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
group x" (coded as "(?(Rx)"), the group number is stored at the start of the
subpattern using the opcode OP_RREF or OP_NRREF (cf OP_NCREF), and a value of
zero for "the whole pattern". For a DEFINE condition, just the single byte
OP_DEF is used (it has no associated data). 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
October 2010
1	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
9	-----------------
10
11	Many years ago I implemented some regular expression functions to an algorithm
12	suggested by Martin Richards. These were not Unix-like in form, and were quite
13	restricted in what they could do by comparison with Perl. The interesting part
14	about the algorithm was that the amount of space required to hold the compiled
15	form of an expression was known in advance. The code to apply an expression did
16	not operate by backtracking, as the original Henry Spencer code and current
17	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
19	subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA
20	algorithm", though it was not a traditional Finite State Machine (FSM). When
21	the pattern was all used up, all remaining states were possible matches, and
22	the one matching the longest subset of the subject string was chosen. This did
23	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
28	-----------------
29
30	By contrast, the code originally written by Henry Spencer (which was
31	subsequently heavily modified for Perl) compiles the expression twice: once in
32	a dummy mode in order to find out how much store will be needed, and then for
33	real. (The Perl version probably doesn't do this any more; I'm talking about
34	the original library.) The execution function operates by backtracking and
35	maximizing (or, optionally, minimizing in Perl) the amount of the subject that
36	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
41	-------------------------
42
43	For the set of functions that form the "basic" PCRE library (which are
44	unrelated to those mentioned above), I tried at first to invent an algorithm
45	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
47	complexity in Perl regular expressions, I couldn't do this. In any case, a
48	first pass through the pattern is helpful for other reasons.
49
50
51	Computing the memory requirement: how it was
52	--------------------------------------------
53
54	Up to and including release 6.7, PCRE worked by running a very degenerate first
55	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 re-factored 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
85	-----------------------------
86
87	The "traditional", and original, matching function is called pcre_exec(), and
88	it implements an NFA algorithm, similar to the original Henry Spencer algorithm
89	and the way that Perl works. This is not surprising, since it is intended to be
90	as compatible with Perl as possible. This is the function most users of PCRE
91	will use most of the time.
92
93
94	Supplementary matching function
95	-------------------------------
96
97	From PCRE 6.0, there is also a supplementary matching function called
98	pcre_dfa_exec(). This implements a DFA matching algorithm that searches
99	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
101	intreprets the same compiled pattern data as pcre_exec(); however, not all the
102	facilities are available, and those that are do not always work in quite the
103	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	Format of compiled patterns
112	---------------------------
113
114	The compiled form of a pattern is a vector of bytes, containing items of
115	variable length. The first byte in an item is an opcode, and the length of the
116	item is either implicit in the opcode or contained in the data bytes that
117	follow it.
118
119	In many cases below LINK_SIZE data values are specified for offsets within the
120	compiled pattern. The default value for LINK_SIZE is 2, but PCRE can be
121	compiled to use 3-byte or 4-byte values for these offsets (impairing the
122	performance). This is necessary only when patterns whose compiled length is
123	greater than 64K are going to be processed. In this description, we assume the
124	"normal" compilation options. Data values that are counts (e.g. for
125	quantifiers) are always just two bytes long.
126
127	A list of the opcodes follows:
128
129	Opcodes with no following data
130	------------------------------
131
132	These items are all just one byte long
133
134	OP_END end of pattern
135	OP_ANY match any one character other than newline
136	OP_ALLANY match any one character, including newline
137	OP_ANYBYTE match any single byte, even in UTF-8 mode
138	OP_SOD match start of data: \A
139	OP_SOM, start of match (subject + offset): \G
140	OP_SET_SOM, set start of match (\K)
141	OP_CIRC ^ (start of data, or after \n in multiline)
142	OP_NOT_WORD_BOUNDARY \W
143	OP_WORD_BOUNDARY \w
144	OP_NOT_DIGIT \D
145	OP_DIGIT \d
146	OP_NOT_HSPACE \H
147	OP_HSPACE \h
148	OP_NOT_WHITESPACE \S
149	OP_WHITESPACE \s
150	OP_NOT_VSPACE \V
151	OP_VSPACE \v
152	OP_NOT_WORDCHAR \W
153	OP_WORDCHAR \w
154	OP_EODN match end of data or \n at end: \Z
155	OP_EOD match end of data: \z
156	OP_DOLL $ (end of data, or before \n in multiline)
157	OP_EXTUNI match an extended Unicode character
158	OP_ANYNL match any Unicode newline sequence
159
160	OP_ACCEPT ) These are Perl 5.10's "backtracking control
161	OP_COMMIT ) verbs". If OP_ACCEPT is inside capturing
162	OP_FAIL ) parentheses, it may be preceded by one or more
163	OP_PRUNE ) OP_CLOSE, followed by a 2-byte number,
164	OP_SKIP ) indicating which parentheses must be closed.
165
166
167	Backtracking control verbs with data
168	------------------------------------
169
170	OP_THEN is followed by a LINK_SIZE offset, which is the distance back to the
171	start of the current branch.
172
173	OP_MARK is followed by the mark name, preceded by a one-byte length, and
174	followed by a binary zero. For (PRUNE), (SKIP), and (*THEN) with arguments,
175	the opcodes OP_PRUNE_ARG, OP_SKIP_ARG, and OP_THEN_ARG are used. For the first
176	two, the name follows immediately; for OP_THEN_ARG, it follows the LINK_SIZE
177	offset value.
178
179
180	Repeating single characters
181	---------------------------
182
183	The common repeats (*, +, ?) when applied to a single character use the
184	following opcodes:
185
186	OP_STAR
187	OP_MINSTAR
188	OP_POSSTAR
189	OP_PLUS
190	OP_MINPLUS
191	OP_POSPLUS
192	OP_QUERY
193	OP_MINQUERY
194	OP_POSQUERY
195
196	In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.
197	Those with "MIN" in their name are the minimizing versions. Those with "POS" in
198	their names are possessive versions. Each is followed by the character that is
199	to be repeated. Other repeats make use of
200
201	OP_UPTO
202	OP_MINUPTO
203	OP_POSUPTO
204	OP_EXACT
205
206	which are followed by a two-byte count (most significant first) and the
207	repeated character. OP_UPTO matches from 0 to the given number. A repeat with a
208	non-zero minimum and a fixed maximum is coded as an OP_EXACT followed by an
209	OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).
210
211
212	Repeating character types
213	-------------------------
214
215	Repeats of things like \d are done exactly as for single characters, except
216	that instead of a character, the opcode for the type is stored in the data
217	byte. The opcodes are:
218
219	OP_TYPESTAR
220	OP_TYPEMINSTAR
221	OP_TYPEPOSSTAR
222	OP_TYPEPLUS
223	OP_TYPEMINPLUS
224	OP_TYPEPOSPLUS
225	OP_TYPEQUERY
226	OP_TYPEMINQUERY
227	OP_TYPEPOSQUERY
228	OP_TYPEUPTO
229	OP_TYPEMINUPTO
230	OP_TYPEPOSUPTO
231	OP_TYPEEXACT
232
233
234	Match by Unicode property
235	-------------------------
236
237	OP_PROP and OP_NOTPROP are used for positive and negative matches of a
238	character by testing its Unicode property (the \p and \P escape sequences).
239	Each is followed by two bytes that encode the desired property as a type and a
240	value.
241
242	Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by
243	three bytes: OP_PROP or OP_NOTPROP and then the desired property type and
244	value.
245
246
247	Matching literal characters
248	---------------------------
249
250	The OP_CHAR opcode is followed by a single character that is to be matched
251	casefully. For caseless matching, OP_CHARNC is used. In UTF-8 mode, the
252	character may be more than one byte long. (Earlier versions of PCRE used
253	multi-character strings, but this was changed to allow some new features to be
254	added.)
255
256
257	Character classes
258	-----------------
259
260	If there is only one character, OP_CHAR or OP_CHARNC is used for a positive
261	class, and OP_NOT for a negative one (that is, for something like [^a]).
262	However, in UTF-8 mode, the use of OP_NOT applies only to characters with
263	values < 128, because OP_NOT is confined to single bytes.
264
265	Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated,
266	negated, single-character class. The normal ones (OP_STAR etc.) are used for a
267	repeated positive single-character class.
268
269	When there's more than one character in a class and all the characters are less
270	than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a negative
271	one. In either case, the opcode is followed by a 32-byte bit map containing a 1
272	bit for every character that is acceptable. The bits are counted from the least
273	significant end of each byte.
274
275	The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,
276	subject characters with values greater than 256 can be handled correctly. For
277	OP_CLASS they don't match, whereas for OP_NCLASS they do.
278
279	For classes containing characters with values > 255, OP_XCLASS is used. It
280	optionally uses a bit map (if any characters lie within it), followed by a list
281	of pairs and single characters. There is a flag character than indicates
282	whether it's a positive or a negative class.
283
284
285	Back references
286	---------------
287
288	OP_REF is followed by two bytes containing the reference number.
289
290
291	Repeating character classes and back references
292	-----------------------------------------------
293
294	Single-character classes are handled specially (see above). This section
295	applies to OP_CLASS and OP_REF. In both cases, the repeat information follows
296	the base item. The matching code looks at the following opcode to see if it is
297	one of
298
299	OP_CRSTAR
300	OP_CRMINSTAR
301	OP_CRPLUS
302	OP_CRMINPLUS
303	OP_CRQUERY
304	OP_CRMINQUERY
305	OP_CRRANGE
306	OP_CRMINRANGE
307
308	All but the last two are just single-byte items. The others are followed by
309	four bytes of data, comprising the minimum and maximum repeat counts. There are
310	no special possessive opcodes for these repeats; a possessive repeat is
311	compiled into an atomic group.
312
313
314	Brackets and alternation
315	------------------------
316
317	A pair of non-capturing (round) brackets is wrapped round each expression at
318	compile time, so alternation always happens in the context of brackets.
319
320	[Note for North Americans: "bracket" to some English speakers, including
321	myself, can be round, square, curly, or pointy. Hence this usage.]
322
323	Non-capturing brackets use the opcode OP_BRA. Originally PCRE was limited to 99
324	capturing brackets and it used a different opcode for each one. From release
325	3.5, the limit was removed by putting the bracket number into the data for
326	higher-numbered brackets. From release 7.0 all capturing brackets are handled
327	this way, using the single opcode OP_CBRA.
328
329	A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
330	next alternative OP_ALT or, if there aren't any branches, to the matching
331	OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
332	the next one, or to the OP_KET opcode. For capturing brackets, the bracket
333	number immediately follows the offset, always as a 2-byte item.
334
335	OP_KET is used for subpatterns that do not repeat indefinitely, while
336	OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
337	maximally respectively. All three are followed by LINK_SIZE bytes giving (as a
338	positive number) the offset back to the matching bracket opcode.
339
340	If a subpattern is quantified such that it is permitted to match zero times, it
341	is preceded by one of OP_BRAZERO, OP_BRAMINZERO, or OP_SKIPZERO. These are
342	single-byte opcodes that tell the matcher that skipping the following
343	subpattern entirely is a valid branch. In the case of the first two, not
344	skipping the pattern is also valid (greedy and non-greedy). The third is used
345	when a pattern has the quantifier {0,0}. It cannot be entirely discarded,
346	because it may be called as a subroutine from elsewhere in the regex.
347
348	A subpattern with an indefinite maximum repetition is replicated in the
349	compiled data its minimum number of times (or once with OP_BRAZERO if the
350	minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
351	as appropriate.
352
353	A subpattern with a bounded maximum repetition is replicated in a nested
354	fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
355	before each replication after the minimum, so that, for example, (abc){2,5} is
356	compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group
357	has the same number.
358
359	When a repeated subpattern has an unbounded upper limit, it is checked to see
360	whether it could match an empty string. If this is the case, the opcode in the
361	final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher
362	that it needs to check for matching an empty string when it hits OP_KETRMIN or
363	OP_KETRMAX, and if so, to break the loop.
364
365
366	Assertions
367	----------
368
369	Forward assertions are just like other subpatterns, but starting with one of
370	the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
371	OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
372	is OP_REVERSE, followed by a two byte count of the number of characters to move
373	back the pointer in the subject string. When operating in UTF-8 mode, the count
374	is a character count rather than a byte count. A separate count is present in
375	each alternative of a lookbehind assertion, allowing them to have different
376	fixed lengths.
377
378
379	Once-only (atomic) subpatterns
380	------------------------------
381
382	These are also just like other subpatterns, but they start with the opcode
383	OP_ONCE. The check for matching an empty string in an unbounded repeat is
384	handled entirely at runtime, so there is just this one opcode.
385
386
387	Conditional subpatterns
388	-----------------------
389
390	These are like other subpatterns, but they start with the opcode OP_COND, or
391	OP_SCOND for one that might match an empty string in an unbounded repeat. If
392	the condition is a back reference, this is stored at the start of the
393	subpattern using the opcode OP_CREF followed by two bytes containing the
394	reference number. OP_NCREF is used instead if the reference was generated by
395	name (so that the runtime code knows to check for duplicate names).
396
397	If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
398	group x" (coded as "(?(Rx)"), the group number is stored at the start of the
399	subpattern using the opcode OP_RREF or OP_NRREF (cf OP_NCREF), and a value of
400	zero for "the whole pattern". For a DEFINE condition, just the single byte
401	OP_DEF is used (it has no associated data). Otherwise, a conditional subpattern
402	always starts with one of the assertions.
403
404
405	Recursion
406	---------
407
408	Recursion either matches the current regex, or some subexpression. The opcode
409	OP_RECURSE is followed by an value which is the offset to the starting bracket
410	from the start of the whole pattern. From release 6.5, OP_RECURSE is
411	automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
412	broke it). OP_RECURSE is also used for "subroutine" calls, even though they
413	are not strictly a recursion.
414
415
416	Callout
417	-------
418
419	OP_CALLOUT is followed by one byte of data that holds a callout number in the
420	range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
421	cases there follows a two-byte value giving the offset in the pattern to the
422	start of the following item, and another two-byte item giving the length of the
423	next item.
424
425
426	Changing options
427	----------------
428
429	If any of the /i, /m, or /s options are changed within a pattern, an OP_OPT
430	opcode is compiled, followed by one byte containing the new settings of these
431	flags. If there are several alternatives, there is an occurrence of OP_OPT at
432	the start of all those following the first options change, to set appropriate
433	options for the start of the alternative. Immediately after the end of the
434	group there is another such item to reset the flags to their previous values. A
435	change of flag right at the very start of the pattern can be handled entirely
436	at compile time, and so does not cause anything to be put into the compiled
437	data.
438
439	Philip Hazel
440	October 2010