perlreguts - Description of the Perl regular expression engine.


   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

   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

   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.

   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

   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 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" 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 */

       Bracketed 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 in the Latin1 range
       are included in the class.

           regnode_charclass        U32 arg1;
                                    char bitmap[ANYOF_BITMAP_SIZE];

       Various flags whose names begin with "ANYOF_" are used for special
       situations.  Above Latin1 matches and things not known until run-
       time are stored in "Perl's pprivate structure".

       There is also a larger form of a char class structure used to
       represent POSIX char classes under "/l" matching, called
       "regnode_charclass_posixl" which has an additional 32-bit bitmap
       indicating which POSIX char classes have been included.

          regnode_charclass_posixl U32 arg1;
                                   char bitmap[ANYOF_BITMAP_SIZE];
                                   U32 classflags;

   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

   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

   What regop is next?

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

   *   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.

   *   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.

   *   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.

   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

   Parsing complications

   The implication of the above description is that a pattern containing
   nested parentheses will result in a call graph which cycles through
   "reg()", "regbranch()", "regpiece()", "regatom()", "reg()",
   "regbranch()" etc multiple times, until the deepest level of nesting is
   reached. All the above routines return a pointer to a "regnode", which
   is usually the last regnode added to the program. However, one
   complication is that reg() returns NULL for parsing "(?:)" syntax for
   embedded modifiers, setting the flag "TRYAGAIN". The "TRYAGAIN"
   propagates upwards until it is captured, in some cases by "regatom()",
   but otherwise unconditionally by "regbranch()". Hence it will never be
   returned by "regbranch()" to "reg()". This flag permits patterns such
   as "(?i)+" to be detected as errors (Quantifier follows nothing in
   regex; marked by <-- HERE in m/(?i)+ <-- HERE /).

   Another complication is that the representation used for the program
   differs if it needs to store Unicode, but it's not always possible to
   know for sure whether it does until midway through parsing. The Unicode
   representation for the program is larger, and cannot be matched as
   efficiently. (See "Unicode and Localisation Support" below for more
   details as to why.)  If the pattern contains literal Unicode, it's
   obvious that the program needs to store Unicode. Otherwise, the parser
   optimistically assumes that the more efficient representation can be
   used, and starts sizing on this basis.  However, if it then encounters
   something in the pattern which must be stored as Unicode, such as an
   "\x{...}" escape sequence representing a character literal, then this
   means that all previously calculated sizes need to be redone, using
   values appropriate for the Unicode representation. Currently, all
   regular expression constructions which can trigger this are parsed by
   code in "regatom()".

   To avoid wasted work when a restart is needed, the sizing pass is
   abandoned - "regatom()" immediately returns NULL, setting the flag
   "RESTART_UTF8".  (This action is encapsulated using the macro
   "REQUIRE_UTF8".) This restart request is propagated up the call chain
   in a similar fashion, until it is "caught" in "Perl_re_op_compile()",
   which marks the pattern as containing Unicode, and restarts the sizing
   pass. It is also possible for constructions within run-time code blocks
   to turn out to need Unicode representation., which is signalled by
   "S_compile_runtime_code()" returning false to "Perl_re_op_compile()".

   The restart was previously implemented using a "longjmp" in "regatom()"
   back to a "setjmp" in "Perl_re_op_compile()", but this proved to be
   problematic as the latter is a large function containing many automatic
   variables, which interact badly with the emergent control flow of

   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

    >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.

    >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[a][rR])(foo|bar)$/"

    >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)
                                     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

      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

   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:


   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:

   *    anchored fixed strings

   *    floating fixed strings

   *    minimum and maximum length requirements

   *    start class

   *    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"

   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 interpreter.

   If we can tell that there is no valid start point then we don't bother
   running the 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

   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.


   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, )
   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.

   Base Structures
   The "regexp" structure described in perlreapi is common to all regex
   engines. Two of its fields 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

   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 by 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 {
            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;

        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

            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

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






   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] <>


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