HACKING - platform/external/pcre - Gitiles

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

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


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

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


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

 By contrast, the code originally written by Henry Spencer (which was
 subsequently heavily modified for Perl) compiles the expression twice: once in
 a dummy mode in order to find out how much store will be needed, and then for
 real. (The Perl version probably doesn't do this any more; I'm talking about
 the original library.) The execution function operates by backtracking and
 maximizing (or, optionally, minimizing in Perl) the amount of the subject that
 matches individual wild portions of the pattern. This is an "NFA algorithm" in
 Friedl's terminology.


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

 For the set of functions that form the "basic" PCRE library (which are
 unrelated to those mentioned above), I tried at first to invent an algorithm
 that used an amount of store bounded by a multiple of the number of characters
 in the pattern, to save on compiling time. However, because of the greater
 complexity in Perl regular expressions, I couldn't do this. In any case, a
 first pass through the pattern is helpful for other reasons.


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

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


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

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

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


 Traditional matching function
 -----------------------------

 The "traditional", and original, matching function is called pcre_exec(), and
 it implements an NFA algorithm, similar to the original Henry Spencer algorithm
 and the way that Perl works. This is not surprising, since it is intended to be
 as compatible with Perl as possible. This is the function most users of PCRE
 will use most of the time.


 Supplementary matching function
 -------------------------------

 From PCRE 6.0, there is also a supplementary matching function called
 pcre_dfa_exec(). This implements a DFA matching algorithm that searches
 simultaneously for all possible matches that start at one point in the subject
 string. (Going back to my roots: see Historical Note 1 above.) This function
 intreprets the same compiled pattern data as pcre_exec(); however, not all the
 facilities are available, and those that are do not always work in quite the
 same way. See the user documentation for details.

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


 Format of compiled patterns
 ---------------------------

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

 In many cases below LINK_SIZE data values are specified for offsets within the
 compiled pattern. The default value for LINK_SIZE is 2, but PCRE can be
 compiled to use 3-byte or 4-byte values for these offsets (impairing the
 performance). This is necessary only when patterns whose compiled length is
 greater than 64K are going to be processed. In this description, we assume the
 "normal" compilation options. Data values that are counts (e.g. for
 quantifiers) are always just two bytes long.

 A list of the opcodes follows:

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

 These items are all just one byte long

   OP_END                 end of pattern
   OP_ANY                 match any one character other than newline
   OP_ALLANY              match any one character, including newline
   OP_ANYBYTE             match any single byte, even in UTF-8 mode
   OP_SOD                 match start of data: \A
   OP_SOM,                start of match (subject + offset): \G
   OP_SET_SOM,            set start of match (\K)
   OP_CIRC                ^ (start of data, or after \n in multiline)
   OP_NOT_WORD_BOUNDARY   \W
   OP_WORD_BOUNDARY       \w
   OP_NOT_DIGIT           \D
   OP_DIGIT               \d
   OP_NOT_HSPACE          \H
   OP_HSPACE              \h
   OP_NOT_WHITESPACE      \S
   OP_WHITESPACE          \s
   OP_NOT_VSPACE          \V
   OP_VSPACE              \v
   OP_NOT_WORDCHAR        \W
   OP_WORDCHAR            \w
   OP_EODN                match end of data or \n at end: \Z
   OP_EOD                 match end of data: \z
   OP_DOLL                $ (end of data, or before \n in multiline)
   OP_EXTUNI              match an extended Unicode character
   OP_ANYNL               match any Unicode newline sequence

   OP_ACCEPT              ) These are Perl 5.10's "backtracking control
   OP_COMMIT              ) verbs". If OP_ACCEPT is inside capturing
   OP_FAIL                ) parentheses, it may be preceded by one or more
   OP_PRUNE               ) OP_CLOSE, followed by a 2-byte number,
   OP_SKIP                ) indicating which parentheses must be closed.


 Backtracking control verbs with data
 ------------------------------------

 OP_THEN is followed by a LINK_SIZE offset, which is the distance back to the
 start of the current branch.

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


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

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

   OP_STAR
   OP_MINSTAR
   OP_POSSTAR
   OP_PLUS
   OP_MINPLUS
   OP_POSPLUS
   OP_QUERY
   OP_MINQUERY
   OP_POSQUERY

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

   OP_UPTO
   OP_MINUPTO
   OP_POSUPTO
   OP_EXACT

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


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

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

   OP_TYPESTAR
   OP_TYPEMINSTAR
   OP_TYPEPOSSTAR
   OP_TYPEPLUS
   OP_TYPEMINPLUS
   OP_TYPEPOSPLUS
   OP_TYPEQUERY
   OP_TYPEMINQUERY
   OP_TYPEPOSQUERY
   OP_TYPEUPTO
   OP_TYPEMINUPTO
   OP_TYPEPOSUPTO
   OP_TYPEEXACT


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

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

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


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

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


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

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

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

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

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

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


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

 OP_REF is followed by two bytes containing the reference number.


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

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

   OP_CRSTAR
   OP_CRMINSTAR
   OP_CRPLUS
   OP_CRMINPLUS
   OP_CRQUERY
   OP_CRMINQUERY
   OP_CRRANGE
   OP_CRMINRANGE

 All but the last two are just single-byte items. The others are followed by
 four bytes of data, comprising the minimum and maximum repeat counts. There are
 no special possessive opcodes for these repeats; a possessive repeat is
 compiled into an atomic group.


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

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

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

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

 A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
 next alternative OP_ALT or, if there aren't any branches, to the matching
 OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
 the next one, or to the OP_KET opcode. For capturing brackets, the bracket
 number immediately follows the offset, always as a 2-byte item.

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

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

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

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

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


 Assertions
 ----------

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


 Once-only (atomic) subpatterns
 ------------------------------

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


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

 These are like other subpatterns, but they start with the opcode OP_COND, or
 OP_SCOND for one that might match an empty string in an unbounded repeat. If
 the condition is a back reference, this is stored at the start of the
 subpattern using the opcode OP_CREF followed by two bytes containing the
 reference number. OP_NCREF is used instead if the reference was generated by
 name (so that the runtime code knows to check for duplicate names).

 If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
 group x" (coded as "(?(Rx)"), the group number is stored at the start of the
 subpattern using the opcode OP_RREF or OP_NRREF (cf OP_NCREF), and a value of
 zero for "the whole pattern". For a DEFINE condition, just the single byte
 OP_DEF is used (it has no associated data). Otherwise, a conditional subpattern
 always starts with one of the assertions.


 Recursion
 ---------

 Recursion either matches the current regex, or some subexpression. The opcode
 OP_RECURSE is followed by an value which is the offset to the starting bracket
 from the start of the whole pattern. From release 6.5, OP_RECURSE is
 automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
 broke it). OP_RECURSE is also used for "subroutine" calls, even though they
 are not strictly a recursion.


 Callout
 -------

 OP_CALLOUT is followed by one byte of data that holds a callout number in the
 range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
 cases there follows a two-byte value giving the offset in the pattern to the
 start of the following item, and another two-byte item giving the length of the
 next item.


 Changing options
 ----------------

 If any of the /i, /m, or /s options are changed within a pattern, an OP_OPT
 opcode is compiled, followed by one byte containing the new settings of these
 flags. If there are several alternatives, there is an occurrence of OP_OPT at
 the start of all those following the first options change, to set appropriate
 options for the start of the alternative. Immediately after the end of the
 group there is another such item to reset the flags to their previous values. A
 change of flag right at the very start of the pattern can be handled entirely
 at compile time, and so does not cause anything to be put into the compiled
 data.

 Philip Hazel
 October 2010
	Technical Notes about PCRE
	--------------------------

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


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

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


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

	By contrast, the code originally written by Henry Spencer (which was
	subsequently heavily modified for Perl) compiles the expression twice: once in
	a dummy mode in order to find out how much store will be needed, and then for
	real. (The Perl version probably doesn't do this any more; I'm talking about
	the original library.) The execution function operates by backtracking and
	maximizing (or, optionally, minimizing in Perl) the amount of the subject that
	matches individual wild portions of the pattern. This is an "NFA algorithm" in
	Friedl's terminology.


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

	For the set of functions that form the "basic" PCRE library (which are
	unrelated to those mentioned above), I tried at first to invent an algorithm
	that used an amount of store bounded by a multiple of the number of characters
	in the pattern, to save on compiling time. However, because of the greater
	complexity in Perl regular expressions, I couldn't do this. In any case, a
	first pass through the pattern is helpful for other reasons.


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

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


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

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

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


	Traditional matching function
	-----------------------------

	The "traditional", and original, matching function is called pcre_exec(), and
	it implements an NFA algorithm, similar to the original Henry Spencer algorithm
	and the way that Perl works. This is not surprising, since it is intended to be
	as compatible with Perl as possible. This is the function most users of PCRE
	will use most of the time.


	Supplementary matching function
	-------------------------------

	From PCRE 6.0, there is also a supplementary matching function called
	pcre_dfa_exec(). This implements a DFA matching algorithm that searches
	simultaneously for all possible matches that start at one point in the subject
	string. (Going back to my roots: see Historical Note 1 above.) This function
	intreprets the same compiled pattern data as pcre_exec(); however, not all the
	facilities are available, and those that are do not always work in quite the
	same way. See the user documentation for details.

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


	Format of compiled patterns
	---------------------------

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

	In many cases below LINK_SIZE data values are specified for offsets within the
	compiled pattern. The default value for LINK_SIZE is 2, but PCRE can be
	compiled to use 3-byte or 4-byte values for these offsets (impairing the
	performance). This is necessary only when patterns whose compiled length is
	greater than 64K are going to be processed. In this description, we assume the
	"normal" compilation options. Data values that are counts (e.g. for
	quantifiers) are always just two bytes long.

	A list of the opcodes follows:

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

	These items are all just one byte long

	OP_END end of pattern
	OP_ANY match any one character other than newline
	OP_ALLANY match any one character, including newline
	OP_ANYBYTE match any single byte, even in UTF-8 mode
	OP_SOD match start of data: \A
	OP_SOM, start of match (subject + offset): \G
	OP_SET_SOM, set start of match (\K)
	OP_CIRC ^ (start of data, or after \n in multiline)
	OP_NOT_WORD_BOUNDARY \W
	OP_WORD_BOUNDARY \w
	OP_NOT_DIGIT \D
	OP_DIGIT \d
	OP_NOT_HSPACE \H
	OP_HSPACE \h
	OP_NOT_WHITESPACE \S
	OP_WHITESPACE \s
	OP_NOT_VSPACE \V
	OP_VSPACE \v
	OP_NOT_WORDCHAR \W
	OP_WORDCHAR \w
	OP_EODN match end of data or \n at end: \Z
	OP_EOD match end of data: \z
	OP_DOLL $ (end of data, or before \n in multiline)
	OP_EXTUNI match an extended Unicode character
	OP_ANYNL match any Unicode newline sequence

	OP_ACCEPT ) These are Perl 5.10's "backtracking control
	OP_COMMIT ) verbs". If OP_ACCEPT is inside capturing
	OP_FAIL ) parentheses, it may be preceded by one or more
	OP_PRUNE ) OP_CLOSE, followed by a 2-byte number,
	OP_SKIP ) indicating which parentheses must be closed.


	Backtracking control verbs with data
	------------------------------------

	OP_THEN is followed by a LINK_SIZE offset, which is the distance back to the
	start of the current branch.

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


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

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

	OP_STAR
	OP_MINSTAR
	OP_POSSTAR
	OP_PLUS
	OP_MINPLUS
	OP_POSPLUS
	OP_QUERY
	OP_MINQUERY
	OP_POSQUERY

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

	OP_UPTO
	OP_MINUPTO
	OP_POSUPTO
	OP_EXACT

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


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

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

	OP_TYPESTAR
	OP_TYPEMINSTAR
	OP_TYPEPOSSTAR
	OP_TYPEPLUS
	OP_TYPEMINPLUS
	OP_TYPEPOSPLUS
	OP_TYPEQUERY
	OP_TYPEMINQUERY
	OP_TYPEPOSQUERY
	OP_TYPEUPTO
	OP_TYPEMINUPTO
	OP_TYPEPOSUPTO
	OP_TYPEEXACT


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

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

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


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

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


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

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

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

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

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

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


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

	OP_REF is followed by two bytes containing the reference number.


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

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

	OP_CRSTAR
	OP_CRMINSTAR
	OP_CRPLUS
	OP_CRMINPLUS
	OP_CRQUERY
	OP_CRMINQUERY
	OP_CRRANGE
	OP_CRMINRANGE

	All but the last two are just single-byte items. The others are followed by
	four bytes of data, comprising the minimum and maximum repeat counts. There are
	no special possessive opcodes for these repeats; a possessive repeat is
	compiled into an atomic group.


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

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

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

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

	A bracket opcode is followed by LINK_SIZE bytes which give the offset to the
	next alternative OP_ALT or, if there aren't any branches, to the matching
	OP_KET opcode. Each OP_ALT is followed by LINK_SIZE bytes giving the offset to
	the next one, or to the OP_KET opcode. For capturing brackets, the bracket
	number immediately follows the offset, always as a 2-byte item.

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

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

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

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

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


	Assertions
	----------

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


	Once-only (atomic) subpatterns
	------------------------------

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


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

	These are like other subpatterns, but they start with the opcode OP_COND, or
	OP_SCOND for one that might match an empty string in an unbounded repeat. If
	the condition is a back reference, this is stored at the start of the
	subpattern using the opcode OP_CREF followed by two bytes containing the
	reference number. OP_NCREF is used instead if the reference was generated by
	name (so that the runtime code knows to check for duplicate names).

	If the condition is "in recursion" (coded as "(?(R)"), or "in recursion of
	group x" (coded as "(?(Rx)"), the group number is stored at the start of the
	subpattern using the opcode OP_RREF or OP_NRREF (cf OP_NCREF), and a value of
	zero for "the whole pattern". For a DEFINE condition, just the single byte
	OP_DEF is used (it has no associated data). Otherwise, a conditional subpattern
	always starts with one of the assertions.


	Recursion
	---------

	Recursion either matches the current regex, or some subexpression. The opcode
	OP_RECURSE is followed by an value which is the offset to the starting bracket
	from the start of the whole pattern. From release 6.5, OP_RECURSE is
	automatically wrapped inside OP_ONCE brackets (because otherwise some patterns
	broke it). OP_RECURSE is also used for "subroutine" calls, even though they
	are not strictly a recursion.


	Callout
	-------

	OP_CALLOUT is followed by one byte of data that holds a callout number in the
	range 0 to 254 for manual callouts, or 255 for an automatic callout. In both
	cases there follows a two-byte value giving the offset in the pattern to the
	start of the following item, and another two-byte item giving the length of the
	next item.


	Changing options
	----------------

	If any of the /i, /m, or /s options are changed within a pattern, an OP_OPT
	opcode is compiled, followed by one byte containing the new settings of these
	flags. If there are several alternatives, there is an occurrence of OP_OPT at
	the start of all those following the first options change, to set appropriate
	options for the start of the alternative. Immediately after the end of the
	group there is another such item to reset the flags to their previous values. A
	change of flag right at the very start of the pattern can be handled entirely
	at compile time, and so does not cause anything to be put into the compiled
	data.

	Philip Hazel
	October 2010