Contents of /code/trunk/HACKING

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

These are very rough technical notes that record potentially useful information 
about PCRE internals. For information about testing PCRE, see the pcretest 
documentation and the comment at the head of the RunTest file.


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 refactored 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. From release 8.20, if PCRE is compiled with 
just-in-time (JIT) support, and studying a compiled pattern with JIT is 
successful, the JIT code is run instead of the normal pcre_exec() code, but the 
result is the same.


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.


Changeable options
------------------

The /i, /m, or /s options (PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL) may
change in the middle of patterns. From PCRE 8.13, their processing is handled
entirely at compile time by generating different opcodes for the different
settings. The runtime functions do not need to keep track of an options state 
any more.


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.

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)
  OP_CIRCM               ^ multiline mode (start of data or after newline)
  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 final newline)
  OP_DOLLM               $ multiline mode (end of data or before newline)
  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 (optional) data
-----------------------------------------------

(*THEN) without an argument generates the opcode OP_THEN and no following data.
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, with the name
following in the same format.
  

Matching literal characters
---------------------------

The OP_CHAR opcode is followed by a single character that is to be matched 
casefully. For caseless matching, OP_CHARI 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.)


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

The common repeats (*, +, ?) when applied to a single character use the
following opcodes, which come in caseful and caseless versions:

  Caseful         Caseless
  OP_STAR         OP_STARI      
  OP_MINSTAR      OP_MINSTARI   
  OP_POSSTAR      OP_POSSTARI   
  OP_PLUS         OP_PLUSI      
  OP_MINPLUS      OP_MINPLUSI   
  OP_POSPLUS      OP_POSPLUSI   
  OP_QUERY        OP_QUERYI     
  OP_MINQUERY     OP_MINQUERYI  
  OP_POSQUERY     OP_POSQUERYI  

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 these opcodes:

  Caseful         Caseless
  OP_UPTO         OP_UPTOI    
  OP_MINUPTO      OP_MINUPTOI 
  OP_POSUPTO      OP_POSUPTOI 
  OP_EXACT        OP_EXACTI   

Each of these is 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.


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

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

Another set of 13 repeating opcodes (called OP_NOTSTAR etc.) are used for a
repeated, negated, single-character class. The normal single-character opcodes
(OP_STAR, etc.) are used for a repeated positive single-character class.

When there is 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. In caseless mode, bits for both 
cases are set.

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 do not 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 (for a range) and single characters. In caseless mode, both cases are 
explicitly listed. There is a flag character than indicates whether it is a
positive or a negative class.


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

OP_REF (caseful) or OP_REFI (caseless) 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[I]. 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 (see below for possessive repetitions). 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.

Possessive brackets
-------------------

When a repeated group (capturing or non-capturing) is marked as possessive by
the "+" notation, e.g. (abc)++, different opcodes are used. Their names all
have POS on the end, e.g. OP_BRAPOS instead of OP_BRA and OP_SCPBRPOS instead 
of OP_SCBRA. The end of such a group is marked by OP_KETRPOS. If the minimum 
repetition is zero, the group is preceded by OP_BRAPOSZERO.


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.


Philip Hazel
October 2011
1	Technical Notes about PCRE
2	--------------------------
3
4	These are very rough technical notes that record potentially useful information
5	about PCRE internals. For information about testing PCRE, see the pcretest
6	documentation and the comment at the head of the RunTest file.
7
8
9	Historical note 1
10	-----------------
11
12	Many years ago I implemented some regular expression functions to an algorithm
13	suggested by Martin Richards. These were not Unix-like in form, and were quite
14	restricted in what they could do by comparison with Perl. The interesting part
15	about the algorithm was that the amount of space required to hold the compiled
16	form of an expression was known in advance. The code to apply an expression did
17	not operate by backtracking, as the original Henry Spencer code and current
18	Perl code does, but instead checked all possibilities simultaneously by keeping
19	a list of current states and checking all of them as it advanced through the
20	subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA
21	algorithm", though it was not a traditional Finite State Machine (FSM). When
22	the pattern was all used up, all remaining states were possible matches, and
23	the one matching the longest subset of the subject string was chosen. This did
24	not necessarily maximize the individual wild portions of the pattern, as is
25	expected in Unix and Perl-style regular expressions.
26
27
28	Historical note 2
29	-----------------
30
31	By contrast, the code originally written by Henry Spencer (which was
32	subsequently heavily modified for Perl) compiles the expression twice: once in
33	a dummy mode in order to find out how much store will be needed, and then for
34	real. (The Perl version probably doesn't do this any more; I'm talking about
35	the original library.) The execution function operates by backtracking and
36	maximizing (or, optionally, minimizing in Perl) the amount of the subject that
37	matches individual wild portions of the pattern. This is an "NFA algorithm" in
38	Friedl's terminology.
39
40
41	OK, here's the real stuff
42	-------------------------
43
44	For the set of functions that form the "basic" PCRE library (which are
45	unrelated to those mentioned above), I tried at first to invent an algorithm
46	that used an amount of store bounded by a multiple of the number of characters
47	in the pattern, to save on compiling time. However, because of the greater
48	complexity in Perl regular expressions, I couldn't do this. In any case, a
49	first pass through the pattern is helpful for other reasons.
50
51
52	Computing the memory requirement: how it was
53	--------------------------------------------
54
55	Up to and including release 6.7, PCRE worked by running a very degenerate first
56	pass to calculate a maximum store size, and then a second pass to do the real
57	compile - which might use a bit less than the predicted amount of memory. The
58	idea was that this would turn out faster than the Henry Spencer code because
59	the first pass is degenerate and the second pass can just store stuff straight
60	into the vector, which it knows is big enough.
61
62
63	Computing the memory requirement: how it is
64	-------------------------------------------
65
66	By the time I was working on a potential 6.8 release, the degenerate first pass
67	had become very complicated and hard to maintain. Indeed one of the early
68	things I did for 6.8 was to fix Yet Another Bug in the memory computation. Then
69	I had a flash of inspiration as to how I could run the real compile function in
70	a "fake" mode that enables it to compute how much memory it would need, while
71	actually only ever using a few hundred bytes of working memory, and without too
72	many tests of the mode that might slow it down. So I refactored the compiling
73	functions to work this way. This got rid of about 600 lines of source. It
74	should make future maintenance and development easier. As this was such a major
75	change, I never released 6.8, instead upping the number to 7.0 (other quite
76	major changes were also present in the 7.0 release).
77
78	A side effect of this work was that the previous limit of 200 on the nesting
79	depth of parentheses was removed. However, there is a downside: pcre_compile()
80	runs more slowly than before (30% or more, depending on the pattern) because it
81	is doing a full analysis of the pattern. My hope was that this would not be a
82	big issue, and in the event, nobody has commented on it.
83
84
85	Traditional matching function
86	-----------------------------
87
88	The "traditional", and original, matching function is called pcre_exec(), and
89	it implements an NFA algorithm, similar to the original Henry Spencer algorithm
90	and the way that Perl works. This is not surprising, since it is intended to be
91	as compatible with Perl as possible. This is the function most users of PCRE
92	will use most of the time. From release 8.20, if PCRE is compiled with
93	just-in-time (JIT) support, and studying a compiled pattern with JIT is
94	successful, the JIT code is run instead of the normal pcre_exec() code, but the
95	result is the same.
96
97
98	Supplementary matching function
99	-------------------------------
100
101	From PCRE 6.0, there is also a supplementary matching function called
102	pcre_dfa_exec(). This implements a DFA matching algorithm that searches
103	simultaneously for all possible matches that start at one point in the subject
104	string. (Going back to my roots: see Historical Note 1 above.) This function
105	intreprets the same compiled pattern data as pcre_exec(); however, not all the
106	facilities are available, and those that are do not always work in quite the
107	same way. See the user documentation for details.
108
109	The algorithm that is used for pcre_dfa_exec() is not a traditional FSM,
110	because it may have a number of states active at one time. More work would be
111	needed at compile time to produce a traditional FSM where only one state is
112	ever active at once. I believe some other regex matchers work this way.
113
114
115	Changeable options
116	------------------
117
118	The /i, /m, or /s options (PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL) may
119	change in the middle of patterns. From PCRE 8.13, their processing is handled
120	entirely at compile time by generating different opcodes for the different
121	settings. The runtime functions do not need to keep track of an options state
122	any more.
123
124
125	Format of compiled patterns
126	---------------------------
127
128	The compiled form of a pattern is a vector of bytes, containing items of
129	variable length. The first byte in an item is an opcode, and the length of the
130	item is either implicit in the opcode or contained in the data bytes that
131	follow it.
132
133	In many cases below LINK_SIZE data values are specified for offsets within the
134	compiled pattern. The default value for LINK_SIZE is 2, but PCRE can be
135	compiled to use 3-byte or 4-byte values for these offsets (impairing the
136	performance). This is necessary only when patterns whose compiled length is
137	greater than 64K are going to be processed. In this description, we assume the
138	"normal" compilation options. Data values that are counts (e.g. for
139	quantifiers) are always just two bytes long.
140
141	Opcodes with no following data
142	------------------------------
143
144	These items are all just one byte long
145
146	OP_END end of pattern
147	OP_ANY match any one character other than newline
148	OP_ALLANY match any one character, including newline
149	OP_ANYBYTE match any single byte, even in UTF-8 mode
150	OP_SOD match start of data: \A
151	OP_SOM, start of match (subject + offset): \G
152	OP_SET_SOM, set start of match (\K)
153	OP_CIRC ^ (start of data)
154	OP_CIRCM ^ multiline mode (start of data or after newline)
155	OP_NOT_WORD_BOUNDARY \W
156	OP_WORD_BOUNDARY \w
157	OP_NOT_DIGIT \D
158	OP_DIGIT \d
159	OP_NOT_HSPACE \H
160	OP_HSPACE \h
161	OP_NOT_WHITESPACE \S
162	OP_WHITESPACE \s
163	OP_NOT_VSPACE \V
164	OP_VSPACE \v
165	OP_NOT_WORDCHAR \W
166	OP_WORDCHAR \w
167	OP_EODN match end of data or \n at end: \Z
168	OP_EOD match end of data: \z
169	OP_DOLL $ (end of data, or before final newline)
170	OP_DOLLM $ multiline mode (end of data or before newline)
171	OP_EXTUNI match an extended Unicode character
172	OP_ANYNL match any Unicode newline sequence
173
174	OP_ACCEPT ) These are Perl 5.10's "backtracking control
175	OP_COMMIT ) verbs". If OP_ACCEPT is inside capturing
176	OP_FAIL ) parentheses, it may be preceded by one or more
177	OP_PRUNE ) OP_CLOSE, followed by a 2-byte number,
178	OP_SKIP ) indicating which parentheses must be closed.
179
180
181	Backtracking control verbs with (optional) data
182	-----------------------------------------------
183
184	(*THEN) without an argument generates the opcode OP_THEN and no following data.
185	OP_MARK is followed by the mark name, preceded by a one-byte length, and
186	followed by a binary zero. For (PRUNE), (SKIP), and (*THEN) with arguments,
187	the opcodes OP_PRUNE_ARG, OP_SKIP_ARG, and OP_THEN_ARG are used, with the name
188	following in the same format.
189
190
191	Matching literal characters
192	---------------------------
193
194	The OP_CHAR opcode is followed by a single character that is to be matched
195	casefully. For caseless matching, OP_CHARI is used. In UTF-8 mode, the
196	character may be more than one byte long. (Earlier versions of PCRE used
197	multi-character strings, but this was changed to allow some new features to be
198	added.)
199
200
201	Repeating single characters
202	---------------------------
203
204	The common repeats (*, +, ?) when applied to a single character use the
205	following opcodes, which come in caseful and caseless versions:
206
207	Caseful Caseless
208	OP_STAR OP_STARI
209	OP_MINSTAR OP_MINSTARI
210	OP_POSSTAR OP_POSSTARI
211	OP_PLUS OP_PLUSI
212	OP_MINPLUS OP_MINPLUSI
213	OP_POSPLUS OP_POSPLUSI
214	OP_QUERY OP_QUERYI
215	OP_MINQUERY OP_MINQUERYI
216	OP_POSQUERY OP_POSQUERYI
217
218	In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.
219	Those with "MIN" in their name are the minimizing versions. Those with "POS" in
220	their names are possessive versions. Each is followed by the character that is
221	to be repeated. Other repeats make use of these opcodes:
222
223	Caseful Caseless
224	OP_UPTO OP_UPTOI
225	OP_MINUPTO OP_MINUPTOI
226	OP_POSUPTO OP_POSUPTOI
227	OP_EXACT OP_EXACTI
228
229	Each of these is followed by a two-byte count (most significant first) and the
230	repeated character. OP_UPTO matches from 0 to the given number. A repeat with a
231	non-zero minimum and a fixed maximum is coded as an OP_EXACT followed by an
232	OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).
233
234
235	Repeating character types
236	-------------------------
237
238	Repeats of things like \d are done exactly as for single characters, except
239	that instead of a character, the opcode for the type is stored in the data
240	byte. The opcodes are:
241
242	OP_TYPESTAR
243	OP_TYPEMINSTAR
244	OP_TYPEPOSSTAR
245	OP_TYPEPLUS
246	OP_TYPEMINPLUS
247	OP_TYPEPOSPLUS
248	OP_TYPEQUERY
249	OP_TYPEMINQUERY
250	OP_TYPEPOSQUERY
251	OP_TYPEUPTO
252	OP_TYPEMINUPTO
253	OP_TYPEPOSUPTO
254	OP_TYPEEXACT
255
256
257	Match by Unicode property
258	-------------------------
259
260	OP_PROP and OP_NOTPROP are used for positive and negative matches of a
261	character by testing its Unicode property (the \p and \P escape sequences).
262	Each is followed by two bytes that encode the desired property as a type and a
263	value.
264
265	Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by
266	three bytes: OP_PROP or OP_NOTPROP and then the desired property type and
267	value.
268
269
270	Character classes
271	-----------------
272
273	If there is only one character, OP_CHAR or OP_CHARI is used for a positive
274	class, and OP_NOT or OP_NOTI for a negative one (that is, for something like
275	[^a]). However, in UTF-8 mode, the use of OP_NOT[I] applies only to characters
276	with values < 128, because OP_NOT[I] is confined to single bytes.
277
278	Another set of 13 repeating opcodes (called OP_NOTSTAR etc.) are used for a
279	repeated, negated, single-character class. The normal single-character opcodes
280	(OP_STAR, etc.) are used for a repeated positive single-character class.
281
282	When there is more than one character in a class and all the characters are
283	less than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a
284	negative one. In either case, the opcode is followed by a 32-byte bit map
285	containing a 1 bit for every character that is acceptable. The bits are counted
286	from the least significant end of each byte. In caseless mode, bits for both
287	cases are set.
288
289	The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,
290	subject characters with values greater than 256 can be handled correctly. For
291	OP_CLASS they do not match, whereas for OP_NCLASS they do.
292
293	For classes containing characters with values > 255, OP_XCLASS is used. It
294	optionally uses a bit map (if any characters lie within it), followed by a list
295	of pairs (for a range) and single characters. In caseless mode, both cases are
296	explicitly listed. There is a flag character than indicates whether it is a
297	positive or a negative class.
298
299
300	Back references
301	---------------
302
303	OP_REF (caseful) or OP_REFI (caseless) is followed by two bytes containing the
304	reference number.
305
306
307	Repeating character classes and back references
308	-----------------------------------------------
309
310	Single-character classes are handled specially (see above). This section
311	applies to OP_CLASS and OP_REF[I]. In both cases, the repeat information
312	follows the base item. The matching code looks at the following opcode to see
313	if it is one of
314
315	OP_CRSTAR
316	OP_CRMINSTAR
317	OP_CRPLUS
318	OP_CRMINPLUS
319	OP_CRQUERY
320	OP_CRMINQUERY
321	OP_CRRANGE
322	OP_CRMINRANGE
323
324	All but the last two are just single-byte items. The others are followed by
325	four bytes of data, comprising the minimum and maximum repeat counts. There are
326	no special possessive opcodes for these repeats; a possessive repeat is
327	compiled into an atomic group.
328
329
330	Brackets and alternation
331	------------------------
332
333	A pair of non-capturing (round) brackets is wrapped round each expression at
334	compile time, so alternation always happens in the context of brackets.
335
336	[Note for North Americans: "bracket" to some English speakers, including
337	myself, can be round, square, curly, or pointy. Hence this usage.]
338
339	Non-capturing brackets use the opcode OP_BRA. Originally PCRE was limited to 99
340	capturing brackets and it used a different opcode for each one. From release
341	3.5, the limit was removed by putting the bracket number into the data for
342	higher-numbered brackets. From release 7.0 all capturing brackets are handled
343	this way, using the single opcode OP_CBRA.
344
345	A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
346	next alternative OP_ALT or, if there aren't any branches, to the matching
347	OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
348	the next one, or to the OP_KET opcode. For capturing brackets, the bracket
349	number immediately follows the offset, always as a 2-byte item.
350
351	OP_KET is used for subpatterns that do not repeat indefinitely, while
352	OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
353	maximally respectively (see below for possessive repetitions). All three are
354	followed by LINK_SIZE bytes giving (as a positive number) the offset back to
355	the matching bracket opcode.
356
357	If a subpattern is quantified such that it is permitted to match zero times, it
358	is preceded by one of OP_BRAZERO, OP_BRAMINZERO, or OP_SKIPZERO. These are
359	single-byte opcodes that tell the matcher that skipping the following
360	subpattern entirely is a valid branch. In the case of the first two, not
361	skipping the pattern is also valid (greedy and non-greedy). The third is used
362	when a pattern has the quantifier {0,0}. It cannot be entirely discarded,
363	because it may be called as a subroutine from elsewhere in the regex.
364
365	A subpattern with an indefinite maximum repetition is replicated in the
366	compiled data its minimum number of times (or once with OP_BRAZERO if the
367	minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
368	as appropriate.
369
370	A subpattern with a bounded maximum repetition is replicated in a nested
371	fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
372	before each replication after the minimum, so that, for example, (abc){2,5} is
373	compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group
374	has the same number.
375
376	When a repeated subpattern has an unbounded upper limit, it is checked to see
377	whether it could match an empty string. If this is the case, the opcode in the
378	final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher
379	that it needs to check for matching an empty string when it hits OP_KETRMIN or
380	OP_KETRMAX, and if so, to break the loop.
381
382	Possessive brackets
383	-------------------
384
385	When a repeated group (capturing or non-capturing) is marked as possessive by
386	the "+" notation, e.g. (abc)++, different opcodes are used. Their names all
387	have POS on the end, e.g. OP_BRAPOS instead of OP_BRA and OP_SCPBRPOS instead
388	of OP_SCBRA. The end of such a group is marked by OP_KETRPOS. If the minimum
389	repetition is zero, the group is preceded by OP_BRAPOSZERO.
390
391
392	Assertions
393	----------
394
395	Forward assertions are just like other subpatterns, but starting with one of
396	the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
397	OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
398	is OP_REVERSE, followed by a two byte count of the number of characters to move
399	back the pointer in the subject string. When operating in UTF-8 mode, the count
400	is a character count rather than a byte count. A separate count is present in
401	each alternative of a lookbehind assertion, allowing them to have different
402	fixed lengths.
403
404
405	Once-only (atomic) subpatterns
406	------------------------------
407
408	These are also just like other subpatterns, but they start with the opcode
409	OP_ONCE. The check for matching an empty string in an unbounded repeat is
410	handled entirely at runtime, so there is just this one opcode.
411
412
413	Conditional subpatterns
414	-----------------------
415
416	These are like other subpatterns, but they start with the opcode OP_COND, or
417	OP_SCOND for one that might match an empty string in an unbounded repeat. If
418	the condition is a back reference, this is stored at the start of the
419	subpattern using the opcode OP_CREF followed by two bytes containing the
420	reference number. OP_NCREF is used instead if the reference was generated by
421	name (so that the runtime code knows to check for duplicate names).
422
423	If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
424	group x" (coded as "(?(Rx)"), the group number is stored at the start of the
425	subpattern using the opcode OP_RREF or OP_NRREF (cf OP_NCREF), and a value of
426	zero for "the whole pattern". For a DEFINE condition, just the single byte
427	OP_DEF is used (it has no associated data). Otherwise, a conditional subpattern
428	always starts with one of the assertions.
429
430
431	Recursion
432	---------
433
434	Recursion either matches the current regex, or some subexpression. The opcode
435	OP_RECURSE is followed by an value which is the offset to the starting bracket
436	from the start of the whole pattern. From release 6.5, OP_RECURSE is
437	automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
438	broke it). OP_RECURSE is also used for "subroutine" calls, even though they
439	are not strictly a recursion.
440
441
442	Callout
443	-------
444
445	OP_CALLOUT is followed by one byte of data that holds a callout number in the
446	range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
447	cases there follows a two-byte value giving the offset in the pattern to the
448	start of the following item, and another two-byte item giving the length of the
449	next item.
450
451
452	Philip Hazel
453	October 2011