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
       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

       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 */

           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

               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

       A set of macros makes accessing the fields easier and more consistent.
       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.

       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
       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

                  | 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

        >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" 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

                                       ~ 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.
       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"

       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 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

       "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
       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:


           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

       There are two structures used to store a compiled regular expression.
       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
                   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" 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
                    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] <>

perl v5.14.2                      2010-12-30                     PERLREGUTS(1)
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