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1  Technical Notes about PCRE  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  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  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  restricted in what they could do by comparison with Perl. The interesting part
# Line 9  form of an expression was known in advan Line 12  form of an expression was known in advan
12  not operate by backtracking, as the original Henry Spencer code and current  not operate by backtracking, as the original Henry Spencer code and current
13  Perl code does, but instead checked all possibilities simultaneously by keeping  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  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  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  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  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  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.  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  By contrast, the code originally written by Henry Spencer and subsequently
25  heavily modified for Perl actually compiles the expression twice: once in a  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  dummy mode in order to find out how much store will be needed, and then for
# Line 23  optionally, minimizing in Perl) the amou Line 29  optionally, minimizing in Perl) the amou
29  individual wild portions of the pattern. This is an "NFA algorithm" in Friedl's  individual wild portions of the pattern. This is an "NFA algorithm" in Friedl's
30  terminology.  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  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  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  of store bounded by a multiple of the number of characters in the pattern, to
# Line 38  got quite big anyway to handle all the P Line 47  got quite big anyway to handle all the P
47    
48  The compiled form of a pattern is a vector of bytes, containing items of  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  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 which  item is either implicit in the opcode or contained in the data bytes that
51  follow it. A list of all the opcodes follows:  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  Opcodes with no following data
62  ------------------------------  ------------------------------
# Line 48  These items are all just one byte long Line 65  These items are all just one byte long
65    
66    OP_END                 end of pattern    OP_END                 end of pattern
67    OP_ANY                 match any character    OP_ANY                 match any character
68    OP_ANYBYTE             match any single byte, even in UTF-8 mode    OP_ANYBYTE             match any single byte, even in UTF-8 mode
69    OP_SOD                 match start of data: \A    OP_SOD                 match start of data: \A
70    OP_SOM,                start of match (subject + offset): \G    OP_SOM,                start of match (subject + offset): \G
71    OP_CIRC                ^ (start of data, or after \n in multiline)    OP_CIRC                ^ (start of data, or after \n in multiline)
# Line 63  These items are all just one byte long Line 80  These items are all just one byte long
80    OP_EODN                match end of data or \n at end: \Z    OP_EODN                match end of data or \n at end: \Z
81    OP_EOD                 match end of data: \z    OP_EOD                 match end of data: \z
82    OP_DOLL                $ (end of data, or before \n in multiline)    OP_DOLL                $ (end of data, or before \n in multiline)
83      OP_EXTUNI              match an extended Unicode character
84    
85    
86  Repeating single characters  Repeating single characters
87  ---------------------------  ---------------------------
88    
89  The common repeats (*, +, ?) when applied to a single character appear as  The common repeats (*, +, ?) when applied to a single character use the
90  two-byte items using the following opcodes:  following opcodes:
91    
92    OP_STAR    OP_STAR
93    OP_MINSTAR    OP_MINSTAR
# Line 78  two-byte items using the following opcod Line 96  two-byte items using the following opcod
96    OP_QUERY    OP_QUERY
97    OP_MINQUERY    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  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  the character that is to be repeated. Other repeats make use of
102    
# Line 109  byte. The opcodes are: Line 128  byte. The opcodes are:
128    OP_TYPEEXACT    OP_TYPEEXACT
129    
130    
131  Matching a character string  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_CHARS opcode is followed by a one-byte count and then that number of  The OP_CHAR opcode is followed by a single character that is to be matched
146  characters. If there are more than 255 characters in sequence, successive  casefully. For caseless matching, OP_CHARNC is used. In UTF-8 mode, the
147  instances of OP_CHARS are used.  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  Character classes
153  -----------------  -----------------
154    
155  If there is only one character, OP_CHARS is used for a positive class,  If there is only one character, OP_CHAR or OP_CHARNC is used for a positive
156  and OP_NOT for a negative one (that is, for something like [^a]). However, in  class, and OP_NOT for a negative one (that is, for something like [^a]).
157  UTF-8 mode, this applies only to characters with values < 128, because OP_NOT  However, in UTF-8 mode, the use of OP_NOT applies only to characters with
158  is confined to single bytes.  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,  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  negated, single-character class. The normal ones (OP_STAR etc.) are used for a
162  repeated positive single-character class.  repeated positive single-character class.
163    
164  When there's more than one character in a class and all the characters are less  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  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  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  bit for every character that is acceptable. The bits are counted from the least
168  significant end of each byte.  significant end of each byte.
169    
170  The reason for having both OP_CLASS and OP_NCLASS is so that, in UTF-8 mode,  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  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.  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  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  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  of pairs and single characters. There is a flag character than indicates
177  whether it's a positive or a negative class.  whether it's a positive or a negative class.
178    
179    
# Line 192  the bracket itself. (They could have all Line 224  the bracket itself. (They could have all
224  minimal changes.)  minimal changes.)
225    
226  A bracket opcode is followed by two bytes which give the offset to the next  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 KET  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,  opcode. Each OP_ALT is followed by two bytes giving the offset to the next one,
229  or to the KET opcode.  or to the OP_KET opcode.
230    
231  OP_KET is used for subpatterns that do not repeat indefinitely, while  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  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  maximally respectively. All three are followed by two bytes giving (as a
234  positive number) the offset back to the matching BRA opcode.  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  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  is preceded by one of OP_BRAZERO or OP_BRAMINZERO. These are single-byte
# Line 207  opcodes which tell the matcher that skip Line 239  opcodes which tell the matcher that skip
239  valid branch.  valid branch.
240    
241  A subpattern with an indefinite maximum repetition is replicated in the  A subpattern with an indefinite maximum repetition is replicated in the
242  compiled data its minimum number of times (or once with a BRAZERO if the  compiled data its minimum number of times (or once with OP_BRAZERO if the
243  minimum is zero), with the final copy terminating with a KETRMIN or KETRMAX as  minimum is zero), with the final copy terminating with OP_KETRMIN or OP_KETRMAX
244  appropriate.  as appropriate.
245    
246  A subpattern with a bounded maximum repetition is replicated in a nested  A subpattern with a bounded maximum repetition is replicated in a nested
247  fashion up to the maximum number of times, with BRAZERO or BRAMINZERO before  fashion up to the maximum number of times, with OP_BRAZERO or OP_BRAMINZERO
248  each replication after the minimum, so that, for example, (abc){2,5} is  before each replication after the minimum, so that, for example, (abc){2,5} is
249  compiled as (abc)(abc)((abc)((abc)(abc)?)?)?. The 99 and 200 bracket limits do  compiled as (abc)(abc)((abc)((abc)(abc)?)?)?.
 not apply to these internally generated brackets.  
250    
251    
252  Assertions  Assertions
# Line 260  from the start of the whole pattern. Line 291  from the start of the whole pattern.
291  Callout  Callout
292  -------  -------
293    
294  OP_CALLOUT is followed by one byte of data that holds a callout number in the  OP_CALLOUT is followed by one byte of data that holds a callout number in the
295  range 0 to 255.  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  Changing options
# Line 278  at compile time, and so does not cause a Line 312  at compile time, and so does not cause a
312  data.  data.
313    
314  Philip Hazel  Philip Hazel
315  August 2003  September 2004
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