FCL/sf/os/fshell: comparison libraries/spcre/libpcre/pcre/HACKING

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+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 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              )
+OP_COMMIT              )
+OP_FAIL                ) These are Perl 5.10's "backtracking
+OP_PRUNE               ) control verbs".
+OP_SKIP                )
+OP_THEN                )
+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. 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
+April 2008