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 are also present in the 7.0 release).

A side effect of this work is 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 is that this is not a big
issue.

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 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 "two-byte" data values are specified. This is in fact just
a default when the number is an offset within the compiled pattern. 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. "Two-byte" data values that are counts (e.g. for 
quantifiers) are always just two bytes.

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_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 
  

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 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)?)?)?, 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. 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, 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
June 2007
1	nigel	41	Technical Notes about PCRE
2			--------------------------
3
4	nigel	91	These are very rough technical notes that record potentially useful information
5			about PCRE internals.
6
7	nigel	75	Historical note 1
8			-----------------
9
10	nigel	41	Many years ago I implemented some regular expression functions to an algorithm
11			suggested by Martin Richards. These were not Unix-like in form, and were quite
12			restricted in what they could do by comparison with Perl. The interesting part
13			about the algorithm was that the amount of space required to hold the compiled
14			form of an expression was known in advance. The code to apply an expression did
15	nigel	63	not operate by backtracking, as the original Henry Spencer code and current
16			Perl code does, but instead checked all possibilities simultaneously by keeping
17			a list of current states and checking all of them as it advanced through the
18	nigel	75	subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA
19	nigel	93	algorithm", though it was not a traditional Finite State Machine (FSM). When
20			the pattern was all used up, all remaining states were possible matches, and
21			the one matching the longest subset of the subject string was chosen. This did
22			not necessarily maximize the individual wild portions of the pattern, as is
23			expected in Unix and Perl-style regular expressions.
24	nigel	41
25	nigel	75	Historical note 2
26			-----------------
27
28	nigel	91	By contrast, the code originally written by Henry Spencer (which was
29			subsequently heavily modified for Perl) compiles the expression twice: once in
30			a dummy mode in order to find out how much store will be needed, and then for
31			real. (The Perl version probably doesn't do this any more; I'm talking about
32			the original library.) The execution function operates by backtracking and
33			maximizing (or, optionally, minimizing in Perl) the amount of the subject that
34			matches individual wild portions of the pattern. This is an "NFA algorithm" in
35			Friedl's terminology.
36	nigel	41
37	nigel	75	OK, here's the real stuff
38			-------------------------
39
40	nigel	77	For the set of functions that form the "basic" PCRE library (which are
41			unrelated to those mentioned above), I tried at first to invent an algorithm
42			that used an amount of store bounded by a multiple of the number of characters
43			in the pattern, to save on compiling time. However, because of the greater
44			complexity in Perl regular expressions, I couldn't do this. In any case, a
45	nigel	93	first pass through the pattern is helpful for other reasons.
46	nigel	41
47	nigel	93	Computing the memory requirement: how it was
48			--------------------------------------------
49
50			Up to and including release 6.7, PCRE worked by running a very degenerate first
51			pass to calculate a maximum store size, and then a second pass to do the real
52			compile - which might use a bit less than the predicted amount of memory. The
53			idea was that this would turn out faster than the Henry Spencer code because
54			the first pass is degenerate and the second pass can just store stuff straight
55			into the vector, which it knows is big enough.
56
57			Computing the memory requirement: how it is
58			-------------------------------------------
59
60			By the time I was working on a potential 6.8 release, the degenerate first pass
61			had become very complicated and hard to maintain. Indeed one of the early
62			things I did for 6.8 was to fix Yet Another Bug in the memory computation. Then
63			I had a flash of inspiration as to how I could run the real compile function in
64			a "fake" mode that enables it to compute how much memory it would need, while
65			actually only ever using a few hundred bytes of working memory, and without too
66			many tests of the mode that might slow it down. So I re-factored the compiling
67			functions to work this way. This got rid of about 600 lines of source. It
68			should make future maintenance and development easier. As this was such a major
69			change, I never released 6.8, instead upping the number to 7.0 (other quite
70			major changes are also present in the 7.0 release).
71
72			A side effect of this work is that the previous limit of 200 on the nesting
73			depth of parentheses was removed. However, there is a downside: pcre_compile()
74			runs more slowly than before (30% or more, depending on the pattern) because it
75			is doing a full analysis of the pattern. My hope is that this is not a big
76			issue.
77
78	nigel	77	Traditional matching function
79			-----------------------------
80
81			The "traditional", and original, matching function is called pcre_exec(), and
82			it implements an NFA algorithm, similar to the original Henry Spencer algorithm
83			and the way that Perl works. Not surprising, since it is intended to be as
84			compatible with Perl as possible. This is the function most users of PCRE will
85			use most of the time.
86
87			Supplementary matching function
88			-------------------------------
89
90			From PCRE 6.0, there is also a supplementary matching function called
91			pcre_dfa_exec(). This implements a DFA matching algorithm that searches
92			simultaneously for all possible matches that start at one point in the subject
93			string. (Going back to my roots: see Historical Note 1 above.) This function
94			intreprets the same compiled pattern data as pcre_exec(); however, not all the
95	nigel	91	facilities are available, and those that are do not always work in quite the
96	nigel	77	same way. See the user documentation for details.
97
98	nigel	93	The algorithm that is used for pcre_dfa_exec() is not a traditional FSM,
99			because it may have a number of states active at one time. More work would be
100			needed at compile time to produce a traditional FSM where only one state is
101			ever active at once. I believe some other regex matchers work this way.
102
103
104	nigel	77	Format of compiled patterns
105			---------------------------
106
107	nigel	41	The compiled form of a pattern is a vector of bytes, containing items of
108			variable length. The first byte in an item is an opcode, and the length of the
109	nigel	75	item is either implicit in the opcode or contained in the data bytes that
110			follow it.
111	nigel	41
112	nigel	75	In many cases below "two-byte" data values are specified. This is in fact just
113	nigel	93	a default when the number is an offset within the compiled pattern. PCRE can be
114			compiled to use 3-byte or 4-byte values for these offsets (impairing the
115	nigel	75	performance). This is necessary only when patterns whose compiled length is
116	nigel	93	greater than 64K are going to be processed. In this description, we assume the
117			"normal" compilation options. "Two-byte" data values that are counts (e.g. for
118			quantifiers) are always just two bytes.
119	nigel	75
120			A list of all the opcodes follows:
121
122	nigel	41	Opcodes with no following data
123			------------------------------
124
125			These items are all just one byte long
126
127			OP_END end of pattern
128			OP_ANY match any character
129	nigel	75	OP_ANYBYTE match any single byte, even in UTF-8 mode
130	nigel	41	OP_SOD match start of data: \A
131	nigel	71	OP_SOM, start of match (subject + offset): \G
132	ph10	181	OP_SET_SOM, set start of match (\K)
133	nigel	41	OP_CIRC ^ (start of data, or after \n in multiline)
134			OP_NOT_WORD_BOUNDARY \W
135			OP_WORD_BOUNDARY \w
136			OP_NOT_DIGIT \D
137			OP_DIGIT \d
138	ph10	181	OP_NOT_HSPACE \H
139			OP_HSPACE \h
140	nigel	41	OP_NOT_WHITESPACE \S
141			OP_WHITESPACE \s
142	ph10	181	OP_NOT_VSPACE \V
143			OP_VSPACE \v
144	nigel	41	OP_NOT_WORDCHAR \W
145			OP_WORDCHAR \w
146			OP_EODN match end of data or \n at end: \Z
147			OP_EOD match end of data: \z
148			OP_DOLL $ (end of data, or before \n in multiline)
149	nigel	75	OP_EXTUNI match an extended Unicode character
150	nigel	93	OP_ANYNL match any Unicode newline sequence
151	nigel	75
152	nigel	41
153			Repeating single characters
154			---------------------------
155
156	nigel	75	The common repeats (*, +, ?) when applied to a single character use the
157			following opcodes:
158	nigel	41
159			OP_STAR
160			OP_MINSTAR
161	nigel	93	OP_POSSTAR
162	nigel	41	OP_PLUS
163			OP_MINPLUS
164	nigel	93	OP_POSPLUS
165	nigel	41	OP_QUERY
166			OP_MINQUERY
167	nigel	93	OP_POSQUERY
168	nigel	41
169	nigel	75	In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.
170	nigel	93	Those with "MIN" in their name are the minimizing versions. Those with "POS" in
171			their names are possessive versions. Each is followed by the character that is
172			to be repeated. Other repeats make use of
173	nigel	41
174			OP_UPTO
175			OP_MINUPTO
176	nigel	93	OP_POSUPTO
177	nigel	41	OP_EXACT
178
179			which are followed by a two-byte count (most significant first) and the
180			repeated character. OP_UPTO matches from 0 to the given number. A repeat with a
181			non-zero minimum and a fixed maximum is coded as an OP_EXACT followed by an
182	nigel	93	OP_UPTO (or OP_MINUPTO or OPT_POSUPTO).
183	nigel	41
184
185			Repeating character types
186			-------------------------
187
188			Repeats of things like \d are done exactly as for single characters, except
189			that instead of a character, the opcode for the type is stored in the data
190			byte. The opcodes are:
191
192			OP_TYPESTAR
193			OP_TYPEMINSTAR
194	nigel	93	OP_TYPEPOSSTAR
195	nigel	41	OP_TYPEPLUS
196			OP_TYPEMINPLUS
197	nigel	93	OP_TYPEPOSPLUS
198	nigel	41	OP_TYPEQUERY
199			OP_TYPEMINQUERY
200	nigel	93	OP_TYPEPOSQUERY
201	nigel	41	OP_TYPEUPTO
202			OP_TYPEMINUPTO
203	nigel	93	OP_TYPEPOSUPTO
204	nigel	41	OP_TYPEEXACT
205
206
207	nigel	75	Match by Unicode property
208			-------------------------
209
210			OP_PROP and OP_NOTPROP are used for positive and negative matches of a
211			character by testing its Unicode property (the \p and \P escape sequences).
212	nigel	91	Each is followed by two bytes that encode the desired property as a type and a
213			value.
214	nigel	75
215	nigel	91	Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by
216			three bytes: OP_PROP or OP_NOTPROP and then the desired property type and
217			value.
218	nigel	75
219
220			Matching literal characters
221	nigel	41	---------------------------
222
223	nigel	75	The OP_CHAR opcode is followed by a single character that is to be matched
224			casefully. For caseless matching, OP_CHARNC is used. In UTF-8 mode, the
225			character may be more than one byte long. (Earlier versions of PCRE used
226			multi-character strings, but this was changed to allow some new features to be
227			added.)
228	nigel	41
229
230			Character classes
231			-----------------
232
233	nigel	75	If there is only one character, OP_CHAR or OP_CHARNC is used for a positive
234			class, and OP_NOT for a negative one (that is, for something like [^a]).
235			However, in UTF-8 mode, the use of OP_NOT applies only to characters with
236			values < 128, because OP_NOT is confined to single bytes.
237	nigel	41
238	nigel	63	Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated,
239			negated, single-character class. The normal ones (OP_STAR etc.) are used for a
240			repeated positive single-character class.
241
242	nigel	71	When there's more than one character in a class and all the characters are less
243	nigel	75	than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a negative
244	nigel	71	one. In either case, the opcode is followed by a 32-byte bit map containing a 1
245			bit for every character that is acceptable. The bits are counted from the least
246			significant end of each byte.
247	nigel	41
248	nigel	75	The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,
249			subject characters with values greater than 256 can be handled correctly. For
250	nigel	71	OP_CLASS they don't match, whereas for OP_NCLASS they do.
251
252	nigel	63	For classes containing characters with values > 255, OP_XCLASS is used. It
253			optionally uses a bit map (if any characters lie within it), followed by a list
254	nigel	75	of pairs and single characters. There is a flag character than indicates
255	nigel	63	whether it's a positive or a negative class.
256	nigel	41
257	nigel	63
258	nigel	41	Back references
259			---------------
260
261	nigel	53	OP_REF is followed by two bytes containing the reference number.
262	nigel	41
263
264			Repeating character classes and back references
265			-----------------------------------------------
266
267	nigel	93	Single-character classes are handled specially (see above). This section
268			applies to OP_CLASS and OP_REF. In both cases, the repeat information follows
269			the base item. The matching code looks at the following opcode to see if it is
270			one of
271	nigel	41
272			OP_CRSTAR
273			OP_CRMINSTAR
274			OP_CRPLUS
275			OP_CRMINPLUS
276			OP_CRQUERY
277			OP_CRMINQUERY
278			OP_CRRANGE
279			OP_CRMINRANGE
280
281			All but the last two are just single-byte items. The others are followed by
282	nigel	93	four bytes of data, comprising the minimum and maximum repeat counts. There are
283			no special possessive opcodes for these repeats; a possessive repeat is
284			compiled into an atomic group.
285	nigel	41
286
287			Brackets and alternation
288			------------------------
289
290	nigel	43	A pair of non-capturing (round) brackets is wrapped round each expression at
291	nigel	41	compile time, so alternation always happens in the context of brackets.
292	nigel	53
293	nigel	93	[Note for North Americans: "bracket" to some English speakers, including
294			myself, can be round, square, curly, or pointy. Hence this usage.]
295	nigel	41
296	nigel	93	Non-capturing brackets use the opcode OP_BRA. Originally PCRE was limited to 99
297			capturing brackets and it used a different opcode for each one. From release
298			3.5, the limit was removed by putting the bracket number into the data for
299			higher-numbered brackets. From release 7.0 all capturing brackets are handled
300			this way, using the single opcode OP_CBRA.
301	nigel	53
302	nigel	77	A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
303			next alternative OP_ALT or, if there aren't any branches, to the matching
304			OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
305	nigel	93	the next one, or to the OP_KET opcode. For capturing brackets, the bracket
306			number immediately follows the offset, always as a 2-byte item.
307	nigel	41
308			OP_KET is used for subpatterns that do not repeat indefinitely, while
309			OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
310	nigel	77	maximally respectively. All three are followed by LINK_SIZE bytes giving (as a
311	nigel	93	positive number) the offset back to the matching bracket opcode.
312	nigel	41
313			If a subpattern is quantified such that it is permitted to match zero times, it
314			is preceded by one of OP_BRAZERO or OP_BRAMINZERO. These are single-byte
315			opcodes which tell the matcher that skipping this subpattern entirely is a
316			valid branch.
317
318			A subpattern with an indefinite maximum repetition is replicated in the
319	nigel	75	compiled data its minimum number of times (or once with OP_BRAZERO if the
320			minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
321			as appropriate.
322	nigel	41
323			A subpattern with a bounded maximum repetition is replicated in a nested
324	nigel	75	fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
325			before each replication after the minimum, so that, for example, (abc){2,5} is
326	nigel	93	compiled as (abc)(abc)((abc)((abc)(abc)?)?)?, except that each bracketed group
327			has the same number.
328	nigel	41
329	nigel	93	When a repeated subpattern has an unbounded upper limit, it is checked to see
330			whether it could match an empty string. If this is the case, the opcode in the
331			final replication is changed to OP_SBRA or OP_SCBRA. This tells the matcher
332			that it needs to check for matching an empty string when it hits OP_KETRMIN or
333			OP_KETRMAX, and if so, to break the loop.
334	nigel	41
335	nigel	93
336	nigel	41	Assertions
337			----------
338
339			Forward assertions are just like other subpatterns, but starting with one of
340			the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
341			OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
342			is OP_REVERSE, followed by a two byte count of the number of characters to move
343	nigel	49	back the pointer in the subject string. When operating in UTF-8 mode, the count
344			is a character count rather than a byte count. A separate count is present in
345			each alternative of a lookbehind assertion, allowing them to have different
346			fixed lengths.
347	nigel	41
348
349	nigel	93	Once-only (atomic) subpatterns
350			------------------------------
351	nigel	41
352			These are also just like other subpatterns, but they start with the opcode
353	nigel	93	OP_ONCE. The check for matching an empty string in an unbounded repeat is
354			handled entirely at runtime, so there is just this one opcode.
355	nigel	41
356
357			Conditional subpatterns
358			-----------------------
359
360	nigel	93	These are like other subpatterns, but they start with the opcode OP_COND, or
361			OP_SCOND for one that might match an empty string in an unbounded repeat. If
362	nigel	41	the condition is a back reference, this is stored at the start of the
363	nigel	53	subpattern using the opcode OP_CREF followed by two bytes containing the
364	nigel	93	reference number. If the condition is "in recursion" (coded as "(?(R)"), or "in
365			recursion of group x" (coded as "(?(Rx)"), the group number is stored at the
366			start of the subpattern using the opcode OP_RREF, and a value of zero for "the
367			whole pattern". For a DEFINE condition, just the single byte OP_DEF is used (it
368			has no associated data). Otherwise, a conditional subpattern always starts with
369			one of the assertions.
370	nigel	41
371
372	nigel	71	Recursion
373			---------
374
375			Recursion either matches the current regex, or some subexpression. The opcode
376			OP_RECURSE is followed by an value which is the offset to the starting bracket
377	nigel	87	from the start of the whole pattern. From release 6.5, OP_RECURSE is
378			automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
379			broke it). OP_RECURSE is also used for "subroutine" calls, even though they
380			are not strictly a recursion.
381	nigel	71
382
383			Callout
384			-------
385
386	nigel	75	OP_CALLOUT is followed by one byte of data that holds a callout number in the
387			range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
388			cases there follows a two-byte value giving the offset in the pattern to the
389			start of the following item, and another two-byte item giving the length of the
390			next item.
391	nigel	71
392
393	nigel	41	Changing options
394			----------------
395
396	nigel	63	If any of the /i, /m, or /s options are changed within a pattern, an OP_OPT
397			opcode is compiled, followed by one byte containing the new settings of these
398			flags. If there are several alternatives, there is an occurrence of OP_OPT at
399			the start of all those following the first options change, to set appropriate
400			options for the start of the alternative. Immediately after the end of the
401			group there is another such item to reset the flags to their previous values. A
402			change of flag right at the very start of the pattern can be handled entirely
403			at compile time, and so does not cause anything to be put into the compiled
404			data.
405	nigel	41
406			Philip Hazel
407	ph10	181	June 2007