pcre-5.0/doc/Tech.Notes

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

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

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

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

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

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

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

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

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

A list of all the opcodes follows:

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

These items are all just one byte long

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

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

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

  OP_STAR
  OP_MINSTAR
  OP_PLUS
  OP_MINPLUS
  OP_QUERY
  OP_MINQUERY

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

  OP_UPTO
  OP_MINUPTO
  OP_EXACT

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


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

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

  OP_TYPESTAR
  OP_TYPEMINSTAR
  OP_TYPEPLUS
  OP_TYPEMINPLUS
  OP_TYPEQUERY
  OP_TYPEMINQUERY
  OP_TYPEUPTO
  OP_TYPEMINUPTO
  OP_TYPEEXACT


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

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

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


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

The OP_CHAR opcode is followed by a single character that is to be matched 
casefully. For caseless matching, OP_CHARNC is used. In UTF-8 mode, the 
character may be more than one byte long. (Earlier versions of PCRE used 
multi-character strings, but this was changed to allow some new features to be 
added.)


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

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

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

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

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

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


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

OP_REF is followed by two bytes containing the reference number.


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

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

  OP_CRSTAR
  OP_CRMINSTAR
  OP_CRPLUS
  OP_CRMINPLUS
  OP_CRQUERY
  OP_CRMINQUERY
  OP_CRRANGE
  OP_CRMINRANGE

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


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

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

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

Originally PCRE was limited to 99 capturing brackets (so as not to use up all
the opcodes). From release 3.5, there is no limit. What happens is that the
first ones, up to EXTRACT_BASIC_MAX are handled with separate opcodes, as
above. If there are more, the opcode is set to EXTRACT_BASIC_MAX+1, and the
first operation in the bracket is OP_BRANUMBER, followed by a 2-byte bracket
number. This opcode is ignored while matching, but is fished out when handling
the bracket itself. (They could have all been done like this, but I was making
minimal changes.)

A bracket opcode is followed by two 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 two bytes giving the offset to the next one,
or to the OP_KET opcode.

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

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

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

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


Assertions
----------

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


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

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


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

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


Recursion
---------

Recursion either matches the current regex, or some subexpression. The opcode
OP_RECURSE is followed by an value which is the offset to the starting bracket
from the start of the whole pattern.


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
September 2004
1	Technical Notes about PCRE
2	--------------------------
3
4	Historical note 1
5	-----------------
6
7	Many years ago I implemented some regular expression functions to an algorithm
8	suggested by Martin Richards. These were not Unix-like in form, and were quite
9	restricted in what they could do by comparison with Perl. The interesting part
10	about the algorithm was that the amount of space required to hold the compiled
11	form of an expression was known in advance. The code to apply an expression did
12	not operate by backtracking, as the original Henry Spencer code and current
13	Perl code does, but instead checked all possibilities simultaneously by keeping
14	a list of current states and checking all of them as it advanced through the
15	subject string. In the terminology of Jeffrey Friedl's book, it was a "DFA
16	algorithm". When the pattern was all used up, all remaining states were
17	possible matches, and the one matching the longest subset of the subject string
18	was chosen. This did not necessarily maximize the individual wild portions of
19	the pattern, as is expected in Unix and Perl-style regular expressions.
20
21	Historical note 2
22	-----------------
23
24	By contrast, the code originally written by Henry Spencer and subsequently
25	heavily modified for Perl actually compiles the expression twice: once in a
26	dummy mode in order to find out how much store will be needed, and then for
27	real. The execution function operates by backtracking and maximizing (or,
28	optionally, minimizing in Perl) the amount of the subject that matches
29	individual wild portions of the pattern. This is an "NFA algorithm" in Friedl's
30	terminology.
31
32	OK, here's the real stuff
33	-------------------------
34
35	For the set of functions that forms PCRE (which are unrelated to those
36	mentioned above), I tried at first to invent an algorithm that used an amount
37	of store bounded by a multiple of the number of characters in the pattern, to
38	save on compiling time. However, because of the greater complexity in Perl
39	regular expressions, I couldn't do this. In any case, a first pass through the
40	pattern is needed, for a number of reasons. PCRE works by running a very
41	degenerate first pass to calculate a maximum store size, and then a second pass
42	to do the real compile - which may use a bit less than the predicted amount of
43	store. The idea is that this is going to turn out faster because the first pass
44	is degenerate and the second pass can just store stuff straight into the
45	vector. It does make the compiling functions bigger, of course, but they have
46	got quite big anyway to handle all the Perl stuff.
47
48	The compiled form of a pattern is a vector of bytes, containing items of
49	variable length. The first byte in an item is an opcode, and the length of the
50	item is either implicit in the opcode or contained in the data bytes that
51	follow it.
52
53	In many cases below "two-byte" data values are specified. This is in fact just
54	a default. PCRE can be compiled to use 3-byte or 4-byte values (impairing the
55	performance). This is necessary only when patterns whose compiled length is
56	greater than 64K are going to be processed. In this description, we assume the
57	"normal" compilation options.
58
59	A list of all the opcodes follows:
60
61	Opcodes with no following data
62	------------------------------
63
64	These items are all just one byte long
65
66	OP_END end of pattern
67	OP_ANY match any character
68	OP_ANYBYTE match any single byte, even in UTF-8 mode
69	OP_SOD match start of data: \A
70	OP_SOM, start of match (subject + offset): \G
71	OP_CIRC ^ (start of data, or after \n in multiline)
72	OP_NOT_WORD_BOUNDARY \W
73	OP_WORD_BOUNDARY \w
74	OP_NOT_DIGIT \D
75	OP_DIGIT \d
76	OP_NOT_WHITESPACE \S
77	OP_WHITESPACE \s
78	OP_NOT_WORDCHAR \W
79	OP_WORDCHAR \w
80	OP_EODN match end of data or \n at end: \Z
81	OP_EOD match end of data: \z
82	OP_DOLL $ (end of data, or before \n in multiline)
83	OP_EXTUNI match an extended Unicode character
84
85
86	Repeating single characters
87	---------------------------
88
89	The common repeats (*, +, ?) when applied to a single character use the
90	following opcodes:
91
92	OP_STAR
93	OP_MINSTAR
94	OP_PLUS
95	OP_MINPLUS
96	OP_QUERY
97	OP_MINQUERY
98
99	In ASCII mode, these are two-byte items; in UTF-8 mode, the length is variable.
100	Those with "MIN" in their name are the minimizing versions. Each is followed by
101	the character that is to be repeated. Other repeats make use of
102
103	OP_UPTO
104	OP_MINUPTO
105	OP_EXACT
106
107	which are followed by a two-byte count (most significant first) and the
108	repeated character. OP_UPTO matches from 0 to the given number. A repeat with a
109	non-zero minimum and a fixed maximum is coded as an OP_EXACT followed by an
110	OP_UPTO (or OP_MINUPTO).
111
112
113	Repeating character types
114	-------------------------
115
116	Repeats of things like \d are done exactly as for single characters, except
117	that instead of a character, the opcode for the type is stored in the data
118	byte. The opcodes are:
119
120	OP_TYPESTAR
121	OP_TYPEMINSTAR
122	OP_TYPEPLUS
123	OP_TYPEMINPLUS
124	OP_TYPEQUERY
125	OP_TYPEMINQUERY
126	OP_TYPEUPTO
127	OP_TYPEMINUPTO
128	OP_TYPEEXACT
129
130
131	Match by Unicode property
132	-------------------------
133
134	OP_PROP and OP_NOTPROP are used for positive and negative matches of a
135	character by testing its Unicode property (the \p and \P escape sequences).
136	Each is followed by a single byte that encodes the desired property value.
137
138	Repeats of these items use the OP_TYPESTAR etc. set of opcodes, followed by two
139	bytes: OP_PROP or OP_NOTPROP and then the desired property value.
140
141
142	Matching literal characters
143	---------------------------
144
145	The OP_CHAR opcode is followed by a single character that is to be matched
146	casefully. For caseless matching, OP_CHARNC is used. In UTF-8 mode, the
147	character may be more than one byte long. (Earlier versions of PCRE used
148	multi-character strings, but this was changed to allow some new features to be
149	added.)
150
151
152	Character classes
153	-----------------
154
155	If there is only one character, OP_CHAR or OP_CHARNC is used for a positive
156	class, and OP_NOT for a negative one (that is, for something like [^a]).
157	However, in UTF-8 mode, the use of OP_NOT applies only to characters with
158	values < 128, because OP_NOT is confined to single bytes.
159
160	Another set of repeating opcodes (OP_NOTSTAR etc.) are used for a repeated,
161	negated, single-character class. The normal ones (OP_STAR etc.) are used for a
162	repeated positive single-character class.
163
164	When there's more than one character in a class and all the characters are less
165	than 256, OP_CLASS is used for a positive class, and OP_NCLASS for a negative
166	one. In either case, the opcode is followed by a 32-byte bit map containing a 1
167	bit for every character that is acceptable. The bits are counted from the least
168	significant end of each byte.
169
170	The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,
171	subject characters with values greater than 256 can be handled correctly. For
172	OP_CLASS they don't match, whereas for OP_NCLASS they do.
173
174	For classes containing characters with values > 255, OP_XCLASS is used. It
175	optionally uses a bit map (if any characters lie within it), followed by a list
176	of pairs and single characters. There is a flag character than indicates
177	whether it's a positive or a negative class.
178
179
180	Back references
181	---------------
182
183	OP_REF is followed by two bytes containing the reference number.
184
185
186	Repeating character classes and back references
187	-----------------------------------------------
188
189	Single-character classes are handled specially (see above). This applies to
190	OP_CLASS and OP_REF. In both cases, the repeat information follows the base
191	item. The matching code looks at the following opcode to see if it is one of
192
193	OP_CRSTAR
194	OP_CRMINSTAR
195	OP_CRPLUS
196	OP_CRMINPLUS
197	OP_CRQUERY
198	OP_CRMINQUERY
199	OP_CRRANGE
200	OP_CRMINRANGE
201
202	All but the last two are just single-byte items. The others are followed by
203	four bytes of data, comprising the minimum and maximum repeat counts.
204
205
206	Brackets and alternation
207	------------------------
208
209	A pair of non-capturing (round) brackets is wrapped round each expression at
210	compile time, so alternation always happens in the context of brackets.
211
212	Non-capturing brackets use the opcode OP_BRA, while capturing brackets use
213	OP_BRA+1, OP_BRA+2, etc. [Note for North Americans: "bracket" to some English
214	speakers, including myself, can be round, square, curly, or pointy. Hence this
215	usage.]
216
217	Originally PCRE was limited to 99 capturing brackets (so as not to use up all
218	the opcodes). From release 3.5, there is no limit. What happens is that the
219	first ones, up to EXTRACT_BASIC_MAX are handled with separate opcodes, as
220	above. If there are more, the opcode is set to EXTRACT_BASIC_MAX+1, and the
221	first operation in the bracket is OP_BRANUMBER, followed by a 2-byte bracket
222	number. This opcode is ignored while matching, but is fished out when handling
223	the bracket itself. (They could have all been done like this, but I was making
224	minimal changes.)
225
226	A bracket opcode is followed by two bytes which give the offset to the next
227	alternative OP_ALT or, if there aren't any branches, to the matching OP_KET
228	opcode. Each OP_ALT is followed by two bytes giving the offset to the next one,
229	or to the OP_KET opcode.
230
231	OP_KET is used for subpatterns that do not repeat indefinitely, while
232	OP_KETRMIN and OP_KETRMAX are used for indefinite repetitions, minimally or
233	maximally respectively. All three are followed by two bytes giving (as a
234	positive number) the offset back to the matching OP_BRA opcode.
235
236	If a subpattern is quantified such that it is permitted to match zero times, it
237	is preceded by one of OP_BRAZERO or OP_BRAMINZERO. These are single-byte
238	opcodes which tell the matcher that skipping this subpattern entirely is a
239	valid branch.
240
241	A subpattern with an indefinite maximum repetition is replicated in the
242	compiled data its minimum number of times (or once with OP_BRAZERO if the
243	minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
244	as appropriate.
245
246	A subpattern with a bounded maximum repetition is replicated in a nested
247	fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
248	before each replication after the minimum, so that, for example, (abc){2,5} is
249	compiled as (abc)(abc)((abc)((abc)(abc)?)?)?.
250
251
252	Assertions
253	----------
254
255	Forward assertions are just like other subpatterns, but starting with one of
256	the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes
257	OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion
258	is OP_REVERSE, followed by a two byte count of the number of characters to move
259	back the pointer in the subject string. When operating in UTF-8 mode, the count
260	is a character count rather than a byte count. A separate count is present in
261	each alternative of a lookbehind assertion, allowing them to have different
262	fixed lengths.
263
264
265	Once-only subpatterns
266	---------------------
267
268	These are also just like other subpatterns, but they start with the opcode
269	OP_ONCE.
270
271
272	Conditional subpatterns
273	-----------------------
274
275	These are like other subpatterns, but they start with the opcode OP_COND. If
276	the condition is a back reference, this is stored at the start of the
277	subpattern using the opcode OP_CREF followed by two bytes containing the
278	reference number. If the condition is "in recursion" (coded as "(?(R)"), the
279	same scheme is used, with a "reference number" of 0xffff. Otherwise, a
280	conditional subpattern always starts with one of the assertions.
281
282
283	Recursion
284	---------
285
286	Recursion either matches the current regex, or some subexpression. The opcode
287	OP_RECURSE is followed by an value which is the offset to the starting bracket
288	from the start of the whole pattern.
289
290
291	Callout
292	-------
293
294	OP_CALLOUT is followed by one byte of data that holds a callout number in the
295	range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
296	cases there follows a two-byte value giving the offset in the pattern to the
297	start of the following item, and another two-byte item giving the length of the
298	next item.
299
300
301	Changing options
302	----------------
303
304	If any of the /i, /m, or /s options are changed within a pattern, an OP_OPT
305	opcode is compiled, followed by one byte containing the new settings of these
306	flags. If there are several alternatives, there is an occurrence of OP_OPT at
307	the start of all those following the first options change, to set appropriate
308	options for the start of the alternative. Immediately after the end of the
309	group there is another such item to reset the flags to their previous values. A
310	change of flag right at the very start of the pattern can be handled entirely
311	at compile time, and so does not cause anything to be put into the compiled
312	data.
313
314	Philip Hazel
315	September 2004