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PERLREGUTS(1)			 Perl Programmers Reference Guide		    PERLREGUTS(1)

NAME
       perlreguts - Description of the Perl regular expression engine.

DESCRIPTION
       This document is an attempt to shine some light on the guts of the regex engine and how it
       works. The regex engine represents a significant chunk of the perl codebase, but is
       relatively poorly understood. This document is a meagre attempt at addressing this
       situation. It is derived from the author's experience, comments in the source code, other
       papers on the regex engine, feedback on the perl5-porters mail list, and no doubt other
       places as well.

       NOTICE! It should be clearly understood that the behavior and structures discussed in this
       represents the state of the engine as the author understood it at the time of writing. It
       is NOT an API definition, it is purely an internals guide for those who want to hack the
       regex engine, or understand how the regex engine works. Readers of this document are
       expected to understand perl's regex syntax and its usage in detail. If you want to learn
       about the basics of Perl's regular expressions, see perlre. And if you want to replace the
       regex engine with your own, see perlreapi.

OVERVIEW
   A quick note on terms
       There is some debate as to whether to say "regexp" or "regex". In this document we will
       use the term "regex" unless there is a special reason not to, in which case we will
       explain why.

       When speaking about regexes we need to distinguish between their source code form and
       their internal form. In this document we will use the term "pattern" when we speak of
       their textual, source code form, and the term "program" when we speak of their internal
       representation. These correspond to the terms S-regex and B-regex that Mark Jason Dominus
       employs in his paper on "Rx" ([1] in "REFERENCES").

   What is a regular expression engine?
       A regular expression engine is a program that takes a set of constraints specified in a
       mini-language, and then applies those constraints to a target string, and determines
       whether or not the string satisfies the constraints. See perlre for a full definition of
       the language.

       In less grandiose terms, the first part of the job is to turn a pattern into something the
       computer can efficiently use to find the matching point in the string, and the second part
       is performing the search itself.

       To do this we need to produce a program by parsing the text. We then need to execute the
       program to find the point in the string that matches. And we need to do the whole thing
       efficiently.

   Structure of a Regexp Program
       High Level

       Although it is a bit confusing and some people object to the terminology, it is worth
       taking a look at a comment that has been in regexp.h for years:

       This is essentially a linear encoding of a nondeterministic finite-state machine (aka
       syntax charts or "railroad normal form" in parsing technology).

       The term "railroad normal form" is a bit esoteric, with "syntax diagram/charts", or
       "railroad diagram/charts" being more common terms.  Nevertheless it provides a useful
       mental image of a regex program: each node can be thought of as a unit of track, with a
       single entry and in most cases a single exit point (there are pieces of track that fork,
       but statistically not many), and the whole forms a layout with a single entry and single
       exit point. The matching process can be thought of as a car that moves along the track,
       with the particular route through the system being determined by the character read at
       each possible connector point. A car can fall off the track at any point but it may only
       proceed as long as it matches the track.

       Thus the pattern "/foo(?:\w+|\d+|\s+)bar/" can be thought of as the following chart:

			     [start]
				|
			      <foo>
				|
			  +-----+-----+
			  |	|     |
			<\w+> <\d+> <\s+>
			  |	|     |
			  +-----+-----+
				|
			      <bar>
				|
			      [end]

       The truth of the matter is that perl's regular expressions these days are much more
       complex than this kind of structure, but visualising it this way can help when trying to
       get your bearings, and it matches the current implementation pretty closely.

       To be more precise, we will say that a regex program is an encoding of a graph. Each node
       in the graph corresponds to part of the original regex pattern, such as a literal string
       or a branch, and has a pointer to the nodes representing the next component to be matched.
       Since "node" and "opcode" already have other meanings in the perl source, we will call the
       nodes in a regex program "regops".

       The program is represented by an array of "regnode" structures, one or more of which
       represent a single regop of the program. Struct "regnode" is the smallest struct needed,
       and has a field structure which is shared with all the other larger structures.

       The "next" pointers of all regops except "BRANCH" implement concatenation; a "next"
       pointer with a "BRANCH" on both ends of it is connecting two alternatives.  [Here we have
       one of the subtle syntax dependencies: an individual "BRANCH" (as opposed to a collection
       of them) is never concatenated with anything because of operator precedence.]

       The operand of some types of regop is a literal string; for others, it is a regop leading
       into a sub-program.  In particular, the operand of a "BRANCH" node is the first regop of
       the branch.

       NOTE: As the railroad metaphor suggests, this is not a tree structure:  the tail of the
       branch connects to the thing following the set of "BRANCH"es.  It is a like a single line
       of railway track that splits as it goes into a station or railway yard and rejoins as it
       comes out the other side.

       Regops

       The base structure of a regop is defined in regexp.h as follows:

	   struct regnode {
	       U8  flags;    /* Various purposes, sometimes overridden */
	       U8  type;     /* Opcode value as specified by regnodes.h */
	       U16 next_off; /* Offset in size regnode */
	   };

       Other larger "regnode"-like structures are defined in regcomp.h. They are almost like
       subclasses in that they have the same fields as "regnode", with possibly additional fields
       following in the structure, and in some cases the specific meaning (and name) of some of
       base fields are overridden. The following is a more complete description.

       "regnode_1"
       "regnode_2"
	   "regnode_1" structures have the same header, followed by a single four-byte argument;
	   "regnode_2" structures contain two two-byte arguments instead:

	       regnode_1		U32 arg1;
	       regnode_2		U16 arg1;  U16 arg2;

       "regnode_string"
	   "regnode_string" structures, used for literal strings, follow the header with a one-
	   byte length and then the string data. Strings are padded on the end with zero bytes so
	   that the total length of the node is a multiple of four bytes:

	       regnode_string		char string[1];
					U8 str_len; /* overrides flags */

       "regnode_charclass"
	   Character classes are represented by "regnode_charclass" structures, which have a
	   four-byte argument and then a 32-byte (256-bit) bitmap indicating which characters are
	   included in the class.

	       regnode_charclass	U32 arg1;
					char bitmap[ANYOF_BITMAP_SIZE];

       "regnode_charclass_class"
	   There is also a larger form of a char class structure used to represent POSIX char
	   classes called "regnode_charclass_class" which has an additional 4-byte (32-bit)
	   bitmap indicating which POSIX char classes have been included.

	       regnode_charclass_class	U32 arg1;
					char bitmap[ANYOF_BITMAP_SIZE];
					char classflags[ANYOF_CLASSBITMAP_SIZE];

       regnodes.h defines an array called "regarglen[]" which gives the size of each opcode in
       units of "size regnode" (4-byte). A macro is used to calculate the size of an "EXACT" node
       based on its "str_len" field.

       The regops are defined in regnodes.h which is generated from regcomp.sym by regcomp.pl.
       Currently the maximum possible number of distinct regops is restricted to 256, with about
       a quarter already used.

       A set of macros makes accessing the fields easier and more consistent. These include
       "OP()", which is used to determine the type of a "regnode"-like structure; "NEXT_OFF()",
       which is the offset to the next node (more on this later); "ARG()", "ARG1()", "ARG2()",
       "ARG_SET()", and equivalents for reading and setting the arguments; and "STR_LEN()",
       "STRING()" and "OPERAND()" for manipulating strings and regop bearing types.

       What regop is next?

       There are three distinct concepts of "next" in the regex engine, and it is important to
       keep them clear.

       o   There is the "next regnode" from a given regnode, a value which is rarely useful
	   except that sometimes it matches up in terms of value with one of the others, and that
	   sometimes the code assumes this to always be so.

       o   There is the "next regop" from a given regop/regnode. This is the regop physically
	   located after the current one, as determined by the size of the current regop. This is
	   often useful, such as when dumping the structure we use this order to traverse.
	   Sometimes the code assumes that the "next regnode" is the same as the "next regop", or
	   in other words assumes that the sizeof a given regop type is always going to be one
	   regnode large.

       o   There is the "regnext" from a given regop. This is the regop which is reached by
	   jumping forward by the value of "NEXT_OFF()", or in a few cases for longer jumps by
	   the "arg1" field of the "regnode_1" structure. The subroutine "regnext()" handles this
	   transparently.  This is the logical successor of the node, which in some cases, like
	   that of the "BRANCH" regop, has special meaning.

Process Overview
       Broadly speaking, performing a match of a string against a pattern involves the following
       steps:

       A. Compilation
	    1. Parsing for size
	    2. Parsing for construction
	    3. Peep-hole optimisation and analysis
       B. Execution
	    4. Start position and no-match optimisations
	    5. Program execution

       Where these steps occur in the actual execution of a perl program is determined by whether
       the pattern involves interpolating any string variables. If interpolation occurs, then
       compilation happens at run time. If it does not, then compilation is performed at compile
       time. (The "/o" modifier changes this, as does "qr//" to a certain extent.) The engine
       doesn't really care that much.

   Compilation
       This code resides primarily in regcomp.c, along with the header files regcomp.h, regexp.h
       and regnodes.h.

       Compilation starts with "pregcomp()", which is mostly an initialisation wrapper which
       farms work out to two other routines for the heavy lifting: the first is "reg()", which is
       the start point for parsing; the second, "study_chunk()", is responsible for optimisation.

       Initialisation in "pregcomp()" mostly involves the creation and data-filling of a special
       structure, "RExC_state_t" (defined in regcomp.c).  Almost all internally-used routines in
       regcomp.h take a pointer to one of these structures as their first argument, with the name
       "pRExC_state".  This structure is used to store the compilation state and contains many
       fields. Likewise there are many macros which operate on this variable: anything that looks
       like "RExC_xxxx" is a macro that operates on this pointer/structure.

       Parsing for size

       In this pass the input pattern is parsed in order to calculate how much space is needed
       for each regop we would need to emit. The size is also used to determine whether long
       jumps will be required in the program.

       This stage is controlled by the macro "SIZE_ONLY" being set.

       The parse proceeds pretty much exactly as it does during the construction phase, except
       that most routines are short-circuited to change the size field "RExC_size" and not do
       anything else.

       Parsing for construction

       Once the size of the program has been determined, the pattern is parsed again, but this
       time for real. Now "SIZE_ONLY" will be false, and the actual construction can occur.

       "reg()" is the start of the parse process. It is responsible for parsing an arbitrary
       chunk of pattern up to either the end of the string, or the first closing parenthesis it
       encounters in the pattern.  This means it can be used to parse the top-level regex, or any
       section inside of a grouping parenthesis. It also handles the "special parens" that perl's
       regexes have. For instance when parsing "/x(?:foo)y/" "reg()" will at one point be called
       to parse from the "?" symbol up to and including the ")".

       Additionally, "reg()" is responsible for parsing the one or more branches from the
       pattern, and for "finishing them off" by correctly setting their next pointers. In order
       to do the parsing, it repeatedly calls out to "regbranch()", which is responsible for
       handling up to the first "|" symbol it sees.

       "regbranch()" in turn calls "regpiece()" which handles "things" followed by a quantifier.
       In order to parse the "things", "regatom()" is called. This is the lowest level routine,
       which parses out constant strings, character classes, and the various special symbols like
       "$". If "regatom()" encounters a "(" character it in turn calls "reg()".

       The routine "regtail()" is called by both "reg()" and "regbranch()" in order to "set the
       tail pointer" correctly. When executing and we get to the end of a branch, we need to go
       to the node following the grouping parens. When parsing, however, we don't know where the
       end will be until we get there, so when we do we must go back and update the offsets as
       appropriate. "regtail" is used to make this easier.

       A subtlety of the parsing process means that a regex like "/foo/" is originally parsed
       into an alternation with a single branch. It is only afterwards that the optimiser
       converts single branch alternations into the simpler form.

       Parse Call Graph and a Grammar

       The call graph looks like this:

	   reg()			# parse a top level regex, or inside of parens
	       regbranch()		# parse a single branch of an alternation
		   regpiece()		# parse a pattern followed by a quantifier
		       regatom()	# parse a simple pattern
			   regclass()	#   used to handle a class
			   reg()	#   used to handle a parenthesised subpattern
			   ....
		   ...
		   regtail()		# finish off the branch
	       ...
	       regtail()		# finish off the branch sequence. Tie each
					# branch's tail to the tail of the sequence
					# (NEW) In Debug mode this is
					# regtail_study().

       A grammar form might be something like this:

	   atom  : constant | class
	   quant : '*' | '+' | '?' | '{min,max}'
	   _branch: piece
		  | piece _branch
		  | nothing
	   branch: _branch
		 | _branch '|' branch
	   group : '(' branch ')'
	   _piece: atom | group
	   piece : _piece
		 | _piece quant

       Debug Output

       In the 5.9.x development version of perl you can "use re Debug => 'PARSE'" to see some
       trace information about the parse process. We will start with some simple patterns and
       build up to more complex patterns.

       So when we parse "/foo/" we see something like the following table. The left shows what is
       being parsed, and the number indicates where the next regop would go. The stuff on the
       right is the trace output of the graph. The names are chosen to be short to make it less
       dense on the screen. 'tsdy' is a special form of "regtail()" which does some extra
       analysis.

	>foo<		  1    reg
				 brnc
				   piec
				     atom
	><		  4	 tsdy~ EXACT <foo> (EXACT) (1)
				     ~ attach to END(3) offset to 2

       The resulting program then looks like:

	  1: EXACT <foo>(3)
	  3: END(0)

       As you can see, even though we parsed out a branch and a piece, it was ultimately only an
       atom. The final program shows us how things work. We have an "EXACT" regop, followed by an
       "END" regop. The number in parens indicates where the "regnext" of the node goes. The
       "regnext" of an "END" regop is unused, as "END" regops mean we have successfully matched.
       The number on the left indicates the position of the regop in the regnode array.

       Now let's try a harder pattern. We will add a quantifier, so now we have the pattern
       "/foo+/". We will see that "regbranch()" calls "regpiece()" twice.

	>foo+<		  1    reg
				 brnc
				   piec
				     atom
	>o+<		  3	   piec
				     atom
	><		  6	   tail~ EXACT <fo> (1)
			  7	 tsdy~ EXACT <fo> (EXACT) (1)
				     ~ PLUS (END) (3)
				     ~ attach to END(6) offset to 3

       And we end up with the program:

	  1: EXACT <fo>(3)
	  3: PLUS(6)
	  4:   EXACT <o>(0)
	  6: END(0)

       Now we have a special case. The "EXACT" regop has a "regnext" of 0. This is because if it
       matches it should try to match itself again. The "PLUS" regop handles the actual failure
       of the "EXACT" regop and acts appropriately (going to regnode 6 if the "EXACT" matched at
       least once, or failing if it didn't).

       Now for something much more complex: "/x(?:foo*|b[a][rR])(foo|bar)$/"

	>x(?:foo*|b...	  1    reg
				 brnc
				   piec
				     atom
	>(?:foo*|b[...	  3	   piec
				     atom
	>?:foo*|b[a...		       reg
	>foo*|b[a][...			 brnc
					   piec
					     atom
	>o*|b[a][rR...	  5		   piec
					     atom
	>|b[a][rR])...	  8		   tail~ EXACT <fo> (3)
	>b[a][rR])(...	  9		 brnc
			 10		   piec
					     atom
	>[a][rR])(f...	 12		   piec
					     atom
	>a][rR])(fo...			       clas
	>[rR])(foo|...	 14		   tail~ EXACT <b> (10)
					   piec
					     atom
	>rR])(foo|b...			       clas
	>)(foo|bar)...	 25		   tail~ EXACT <a> (12)
					 tail~ BRANCH(3)
			 26		 tsdy~ BRANCH (END) (9)
					     ~ attach to TAIL(25) offset to 16
					 tsdy~ EXACT <fo> (EXACT) (4)
					     ~ STAR (END) (6)
					     ~ attach to TAIL(25) offset to 19
					 tsdy~ EXACT <b> (EXACT) (10)
					     ~ EXACT <a> (EXACT) (12)
					     ~ ANYOF[Rr] (END) (14)
					     ~ attach to TAIL(25) offset to 11
	>(foo|bar)$<		   tail~ EXACT <x> (1)
				   piec
				     atom
	>foo|bar)$<		       reg
			 28		 brnc
					   piec
					     atom
	>|bar)$<	 31		 tail~ OPEN1(26)
	>bar)$< 			 brnc
			 32		   piec
					     atom
	>)$<		 34		 tail~ BRANCH(28)
			 36		 tsdy~ BRANCH (END) (31)
					     ~ attach to CLOSE1(34) offset to 3
					 tsdy~ EXACT <foo> (EXACT) (29)
					     ~ attach to CLOSE1(34) offset to 5
					 tsdy~ EXACT <bar> (EXACT) (32)
					     ~ attach to CLOSE1(34) offset to 2
	>$<			   tail~ BRANCH(3)
				       ~ BRANCH(9)
				       ~ TAIL(25)
				   piec
				     atom
	><		 37	   tail~ OPEN1(26)
				       ~ BRANCH(28)
				       ~ BRANCH(31)
				       ~ CLOSE1(34)
			 38	 tsdy~ EXACT <x> (EXACT) (1)
				     ~ BRANCH (END) (3)
				     ~ BRANCH (END) (9)
				     ~ TAIL (END) (25)
				     ~ OPEN1 (END) (26)
				     ~ BRANCH (END) (28)
				     ~ BRANCH (END) (31)
				     ~ CLOSE1 (END) (34)
				     ~ EOL (END) (36)
				     ~ attach to END(37) offset to 1

       Resulting in the program

	  1: EXACT <x>(3)
	  3: BRANCH(9)
	  4:   EXACT <fo>(6)
	  6:   STAR(26)
	  7:	 EXACT <o>(0)
	  9: BRANCH(25)
	 10:   EXACT <ba>(14)
	 12:   OPTIMIZED (2 nodes)
	 14:   ANYOF[Rr](26)
	 25: TAIL(26)
	 26: OPEN1(28)
	 28:   TRIE-EXACT(34)
	       [StS:1 Wds:2 Cs:6 Uq:5 #Sts:7 Mn:3 Mx:3 Stcls:bf]
		 <foo>
		 <bar>
	 30:   OPTIMIZED (4 nodes)
	 34: CLOSE1(36)
	 36: EOL(37)
	 37: END(0)

       Here we can see a much more complex program, with various optimisations in play. At
       regnode 10 we see an example where a character class with only one character in it was
       turned into an "EXACT" node. We can also see where an entire alternation was turned into a
       "TRIE-EXACT" node. As a consequence, some of the regnodes have been marked as optimised
       away. We can see that the "$" symbol has been converted into an "EOL" regop, a special
       piece of code that looks for "\n" or the end of the string.

       The next pointer for "BRANCH"es is interesting in that it points at where execution should
       go if the branch fails. When executing, if the engine tries to traverse from a branch to a
       "regnext" that isn't a branch then the engine will know that the entire set of branches
       has failed.

       Peep-hole Optimisation and Analysis

       The regular expression engine can be a weighty tool to wield. On long strings and complex
       patterns it can end up having to do a lot of work to find a match, and even more to decide
       that no match is possible.  Consider a situation like the following pattern.

	  'ababababababababababab' =~ /(a|b)*z/

       The "(a|b)*" part can match at every char in the string, and then fail every time because
       there is no "z" in the string. So obviously we can avoid using the regex engine unless
       there is a "z" in the string.  Likewise in a pattern like:

	  /foo(\w+)bar/

       In this case we know that the string must contain a "foo" which must be followed by "bar".
       We can use Fast Boyer-Moore matching as implemented in "fbm_instr()" to find the location
       of these strings. If they don't exist then we don't need to resort to the much more
       expensive regex engine.	Even better, if they do exist then we can use their positions to
       reduce the search space that the regex engine needs to cover to determine if the entire
       pattern matches.

       There are various aspects of the pattern that can be used to facilitate optimisations
       along these lines:

       o    anchored fixed strings

       o    floating fixed strings

       o    minimum and maximum length requirements

       o    start class

       o    Beginning/End of line positions

       Another form of optimisation that can occur is the post-parse "peep-hole" optimisation,
       where inefficient constructs are replaced by more efficient constructs. The "TAIL" regops
       which are used during parsing to mark the end of branches and the end of groups are
       examples of this. These regops are used as place-holders during construction and "always
       match" so they can be "optimised away" by making the things that point to the "TAIL" point
       to the thing that "TAIL" points to, thus "skipping" the node.

       Another optimisation that can occur is that of ""EXACT" merging" which is where two
       consecutive "EXACT" nodes are merged into a single regop. An even more aggressive form of
       this is that a branch sequence of the form "EXACT BRANCH ... EXACT" can be converted into
       a "TRIE-EXACT" regop.

       All of this occurs in the routine "study_chunk()" which uses a special structure
       "scan_data_t" to store the analysis that it has performed, and does the "peep-hole"
       optimisations as it goes.

       The code involved in "study_chunk()" is extremely cryptic. Be careful. :-)

   Execution
       Execution of a regex generally involves two phases, the first being finding the start
       point in the string where we should match from, and the second being running the regop
       interpreter.

       If we can tell that there is no valid start point then we don't bother running interpreter
       at all. Likewise, if we know from the analysis phase that we cannot detect a short-cut to
       the start position, we go straight to the interpreter.

       The two entry points are "re_intuit_start()" and "pregexec()". These routines have a
       somewhat incestuous relationship with overlap between their functions, and "pregexec()"
       may even call "re_intuit_start()" on its own. Nevertheless other parts of the perl source
       code may call into either, or both.

       Execution of the interpreter itself used to be recursive, but thanks to the efforts of
       Dave Mitchell in the 5.9.x development track, that has changed: now an internal stack is
       maintained on the heap and the routine is fully iterative. This can make it tricky as the
       code is quite conservative about what state it stores, with the result that two
       consecutive lines in the code can actually be running in totally different contexts due to
       the simulated recursion.

       Start position and no-match optimisations

       "re_intuit_start()" is responsible for handling start points and no-match optimisations as
       determined by the results of the analysis done by "study_chunk()" (and described in "Peep-
       hole Optimisation and Analysis").

       The basic structure of this routine is to try to find the start- and/or end-points of
       where the pattern could match, and to ensure that the string is long enough to match the
       pattern. It tries to use more efficient methods over less efficient methods and may
       involve considerable cross-checking of constraints to find the place in the string that
       matches.  For instance it may try to determine that a given fixed string must be not only
       present but a certain number of chars before the end of the string, or whatever.

       It calls several other routines, such as "fbm_instr()" which does Fast Boyer Moore
       matching and "find_byclass()" which is responsible for finding the start using the first
       mandatory regop in the program.

       When the optimisation criteria have been satisfied, "reg_try()" is called to perform the
       match.

       Program execution

       "pregexec()" is the main entry point for running a regex. It contains support for
       initialising the regex interpreter's state, running "re_intuit_start()" if needed, and
       running the interpreter on the string from various start positions as needed. When it is
       necessary to use the regex interpreter "pregexec()" calls "regtry()".

       "regtry()" is the entry point into the regex interpreter. It expects as arguments a
       pointer to a "regmatch_info" structure and a pointer to a string.  It returns an integer 1
       for success and a 0 for failure.  It is basically a set-up wrapper around "regmatch()".

       "regmatch" is the main "recursive loop" of the interpreter. It is basically a giant switch
       statement that implements a state machine, where the possible states are the regops
       themselves, plus a number of additional intermediate and failure states. A few of the
       states are implemented as subroutines but the bulk are inline code.

MISCELLANEOUS
   Unicode and Localisation Support
       When dealing with strings containing characters that cannot be represented using an eight-
       bit character set, perl uses an internal representation that is a permissive version of
       Unicode's UTF-8 encoding[2]. This uses single bytes to represent characters from the ASCII
       character set, and sequences of two or more bytes for all other characters. (See
       perlunitut for more information about the relationship between UTF-8 and perl's encoding,
       utf8. The difference isn't important for this discussion.)

       No matter how you look at it, Unicode support is going to be a pain in a regex engine.
       Tricks that might be fine when you have 256 possible characters often won't scale to
       handle the size of the UTF-8 character set.  Things you can take for granted with ASCII
       may not be true with Unicode. For instance, in ASCII, it is safe to assume that
       "sizeof(char1) == sizeof(char2)", but in UTF-8 it isn't. Unicode case folding is vastly
       more complex than the simple rules of ASCII, and even when not using Unicode but only
       localised single byte encodings, things can get tricky (for example, LATIN SMALL LETTER
       SHARP S (U+00DF, ss) should match 'SS' in localised case-insensitive matching).

       Making things worse is that UTF-8 support was a later addition to the regex engine (as it
       was to perl) and this necessarily  made things a lot more complicated. Obviously it is
       easier to design a regex engine with Unicode support in mind from the beginning than it is
       to retrofit it to one that wasn't.

       Nearly all regops that involve looking at the input string have two cases, one for UTF-8,
       and one not. In fact, it's often more complex than that, as the pattern may be UTF-8 as
       well.

       Care must be taken when making changes to make sure that you handle UTF-8 properly, both
       at compile time and at execution time, including when the string and pattern are
       mismatched.

       The following comment in regcomp.h gives an example of exactly how tricky this can be:

	   Two problematic code points in Unicode casefolding of EXACT nodes:

	   U+0390 - GREEK SMALL LETTER IOTA WITH DIALYTIKA AND TONOS
	   U+03B0 - GREEK SMALL LETTER UPSILON WITH DIALYTIKA AND TONOS

	   which casefold to

	   Unicode			UTF-8

	   U+03B9 U+0308 U+0301 	0xCE 0xB9 0xCC 0x88 0xCC 0x81
	   U+03C5 U+0308 U+0301 	0xCF 0x85 0xCC 0x88 0xCC 0x81

	   This means that in case-insensitive matching (or "loose matching",
	   as Unicode calls it), an EXACTF of length six (the UTF-8 encoded
	   byte length of the above casefolded versions) can match a target
	   string of length two (the byte length of UTF-8 encoded U+0390 or
	   U+03B0). This would rather mess up the minimum length computation.

	   What we'll do is to look for the tail four bytes, and then peek
	   at the preceding two bytes to see whether we need to decrease
	   the minimum length by four (six minus two).

	   Thanks to the design of UTF-8, there cannot be false matches:
	   A sequence of valid UTF-8 bytes cannot be a subsequence of
	   another valid sequence of UTF-8 bytes.

   Base Structures
       The "regexp" structure described in perlreapi is common to all regex engines. Two of its
       fields that are intended for the private use of the regex engine that compiled the
       pattern. These are the "intflags" and pprivate members. The "pprivate" is a void pointer
       to an arbitrary structure whose use and management is the responsibility of the compiling
       engine. perl will never modify either of these values. In the case of the stock engine the
       structure pointed to by "pprivate" is called "regexp_internal".

       Its "pprivate" and "intflags" fields contain data specific to each engine.

       There are two structures used to store a compiled regular expression.  One, the "regexp"
       structure described in perlreapi is populated by the engine currently being. used and some
       of its fields read by perl to implement things such as the stringification of "qr//".

       The other structure is pointed to be the "regexp" struct's "pprivate" and is in addition
       to "intflags" in the same struct considered to be the property of the regex engine which
       compiled the regular expression;

       The regexp structure contains all the data that perl needs to be aware of to properly work
       with the regular expression. It includes data about optimisations that perl can use to
       determine if the regex engine should really be used, and various other control info that
       is needed to properly execute patterns in various contexts such as is the pattern anchored
       in some way, or what flags were used during the compile, or whether the program contains
       special constructs that perl needs to be aware of.

       In addition it contains two fields that are intended for the private use of the regex
       engine that compiled the pattern. These are the "intflags" and pprivate members. The
       "pprivate" is a void pointer to an arbitrary structure whose use and management is the
       responsibility of the compiling engine. perl will never modify either of these values.

       As mentioned earlier, in the case of the default engines, the "pprivate" will be a pointer
       to a regexp_internal structure which holds the compiled program and any additional data
       that is private to the regex engine implementation.

       Perl's "pprivate" structure

       The following structure is used as the "pprivate" struct by perl's regex engine. Since it
       is specific to perl it is only of curiosity value to other engine implementations.

	   typedef struct regexp_internal {
		   regexp_paren_ofs *swap; /* Swap copy of *startp / *endp */
		   U32 *offsets;	   /* offset annotations 20001228 MJD
					      data about mapping the program to the
					      string*/
		   regnode *regstclass;    /* Optional startclass as identified or constructed
					      by the optimiser */
		   struct reg_data *data;  /* Additional miscellaneous data used by the program.
					      Used to make it easier to clone and free arbitrary
					      data that the regops need. Often the ARG field of
					      a regop is an index into this structure */
		   regnode program[1];	   /* Unwarranted chumminess with compiler. */
	   } regexp_internal;

       "swap"
	    "swap" formerly was an extra set of startp/endp stored in a "regexp_paren_ofs"
	    struct. This was used when the last successful match was from the same pattern as the
	    current pattern, so that a partial match didn't overwrite the previous match's
	    results, but it caused a problem with re-entrant code such as trying to build the
	    UTF-8 swashes.  Currently unused and left for backward compatibility with 5.10.0.

       "offsets"
	    Offsets holds a mapping of offset in the "program" to offset in the "precomp" string.
	    This is only used by ActiveState's visual regex debugger.

       "regstclass"
	    Special regop that is used by "re_intuit_start()" to check if a pattern can match at
	    a certain position. For instance if the regex engine knows that the pattern must
	    start with a 'Z' then it can scan the string until it finds one and then launch the
	    regex engine from there. The routine that handles this is called "find_by_class()".
	    Sometimes this field points at a regop embedded in the program, and sometimes it
	    points at an independent synthetic regop that has been constructed by the optimiser.

       "data"
	    This field points at a reg_data structure, which is defined as follows

		struct reg_data {
		    U32 count;
		    U8 *what;
		    void* data[1];
		};

	    This structure is used for handling data structures that the regex engine needs to
	    handle specially during a clone or free operation on the compiled product. Each
	    element in the data array has a corresponding element in the what array. During
	    compilation regops that need special structures stored will add an element to each
	    array using the add_data() routine and then store the index in the regop.

       "program"
	    Compiled program. Inlined into the structure so the entire struct can be treated as a
	    single blob.

SEE ALSO
       perlreapi

       perlre

       perlunitut

AUTHOR
       by Yves Orton, 2006.

       With excerpts from Perl, and contributions and suggestions from Ronald J. Kimball, Dave
       Mitchell, Dominic Dunlop, Mark Jason Dominus, Stephen McCamant, and David Landgren.

LICENCE
       Same terms as Perl.

REFERENCES
       [1] <http://perl.plover.com/Rx/paper/>

       [2] <http://www.unicode.org>

perl v5.16.3				    2013-03-04				    PERLREGUTS(1)
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