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perlreguts (1)


perlreguts - Description of the Perl regular expression engine.


Please see following description for synopsis


Perl Programmers Reference Guide                    PERLREGUTS(1)

     perlreguts - Description of the Perl regular expression

     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.

  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.

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     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

  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:

                        |     |     |
                      <\w+> <\d+> <\s+>
                        |     |     |

     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.

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     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.


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

         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.


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         "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" 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 */

         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];

         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 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

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     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

     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

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.

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     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

     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

     "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 ")".

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     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

     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:

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         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
      ><                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.

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      >foo+<            1    reg
      >o+<              3        piec
      ><                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...    1    reg
      >(?:foo*|b[...    3        piec
      >?:foo*|b[a...                 reg
      >foo*|b[a][...                   brnc
      >o*|b[a][rR...    5                piec
      >|b[a][rR])...    8                tail~ EXACT <fo> (3)
      >b[a][rR])(...    9              brnc
                       10                piec
      >[a][rR])(f...   12                piec
      >a][rR])(fo...                         clas
      >[rR])(foo|...   14                tail~ EXACT <b> (10)
      >rR])(foo|b...                         clas
      >)(foo|bar)...   25                tail~ EXACT <a> (12)

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                                       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)$<               tail~ EXACT <x> (1)
      >foo|bar)$<                    reg
                       28              brnc
      >|bar)$<         31              tail~ OPEN1 (26)
      >bar)$<                          brnc
                       32                piec
      >)$<             34              tail~ BRANCH (28)
                       36              tsdy~ BRANCH (END) (31)
                                           ~ attach to CLOSE1 (34) offset to 3
                                       tsdy~ EXACT <foo> (EXACT) (29)
                                           ~ attach to CLOSE1 (34) offset to 5
                                       tsdy~ EXACT <bar> (EXACT) (32)
                                           ~ attach to CLOSE1 (34) offset to 2
      >$<                        tail~ BRANCH (3)
                                     ~ BRANCH (9)
                                     ~ TAIL (25)
      ><               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

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        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]
       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:

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     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 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

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     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

     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

     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

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     "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.

  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.

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Perl Programmers Reference Guide                    PERLREGUTS(1)

     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

     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:


         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.

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Perl Programmers Reference Guide                    PERLREGUTS(1)

     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

     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
                 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. */

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Perl Programmers Reference Guide                    PERLREGUTS(1)

         } regexp_internal;

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

          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.

          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.

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

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Perl Programmers Reference Guide                    PERLREGUTS(1)

     See attributes(5) for descriptions of the following

     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |



     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.

     Same terms as Perl.

     [1] <>

     [2] <>

     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

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