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NAME

       perldebguts - Guts of Perl debugging

DESCRIPTION

       This is not perldebug, which tells you how to use the debugger.  This manpage describes
       low-level details concerning the debugger's internals, which range from difficult to
       impossible to understand for anyone who isn't incredibly intimate with Perl's guts.
       Caveat lector.

Debugger Internals

       Perl has special debugging hooks at compile-time and run-time used to create debugging
       environments.  These hooks are not to be confused with the perl -Dxxx command described in
       perlrun, which is usable only if a special Perl is built per the instructions in the
       INSTALL podpage in the Perl source tree.

       For example, whenever you call Perl's built-in "caller" function from the package "DB",
       the arguments that the corresponding stack frame was called with are copied to the
       @DB::args array.  These mechanisms are enabled by calling Perl with the -d switch.
       Specifically, the following additional features are enabled (cf. "$^P" in perlvar):

       ·   Perl inserts the contents of $ENV{PERL5DB} (or "BEGIN {require 'perl5db.pl'}" if not
           present) before the first line of your program.

       ·   Each array "@{"_<$filename"}" holds the lines of $filename for a file compiled by
           Perl.  The same is also true for "eval"ed strings that contain subroutines, or which
           are currently being executed.  The $filename for "eval"ed strings looks like "(eval
           34)".

           Values in this array are magical in numeric context: they compare equal to zero only
           if the line is not breakable.

       ·   Each hash "%{"_<$filename"}" contains breakpoints and actions keyed by line number.
           Individual entries (as opposed to the whole hash) are settable.  Perl only cares about
           Boolean true here, although the values used by perl5db.pl have the form
           "$break_condition\0$action".

           The same holds for evaluated strings that contain subroutines, or which are currently
           being executed.  The $filename for "eval"ed strings looks like "(eval 34)".

       ·   Each scalar "${"_<$filename"}" contains "_<$filename".  This is also the case for
           evaluated strings that contain subroutines, or which are currently being executed.
           The $filename for "eval"ed strings looks like "(eval 34)".

       ·   After each "require"d file is compiled, but before it is executed,
           "DB::postponed(*{"_<$filename"})" is called if the subroutine "DB::postponed" exists.
           Here, the $filename is the expanded name of the "require"d file, as found in the
           values of %INC.

       ·   After each subroutine "subname" is compiled, the existence of $DB::postponed{subname}
           is checked.  If this key exists, "DB::postponed(subname)" is called if the
           "DB::postponed" subroutine also exists.

       ·   A hash %DB::sub is maintained, whose keys are subroutine names and whose values have
           the form "filename:startline-endline".  "filename" has the form "(eval 34)" for
           subroutines defined inside "eval"s.

       ·   When the execution of your program reaches a point that can hold a breakpoint, the
           "DB::DB()" subroutine is called if any of the variables $DB::trace, $DB::single, or
           $DB::signal is true.  These variables are not "local"izable.  This feature is disabled
           when executing inside "DB::DB()", including functions called from it unless "$^D &
           (1<<30)" is true.

       ·   When execution of the program reaches a subroutine call, a call to &DB::sub(args) is
           made instead, with $DB::sub holding the name of the called subroutine. (This doesn't
           happen if the subroutine was compiled in the "DB" package.)

       Note that if &DB::sub needs external data for it to work, no subroutine call is possible
       without it. As an example, the standard debugger's &DB::sub depends on the $DB::deep
       variable (it defines how many levels of recursion deep into the debugger you can go before
       a mandatory break).  If $DB::deep is not defined, subroutine calls are not possible, even
       though &DB::sub exists.

   Writing Your Own Debugger
       Environment Variables

       The "PERL5DB" environment variable can be used to define a debugger.  For example, the
       minimal "working" debugger (it actually doesn't do anything) consists of one line:

         sub DB::DB {}

       It can easily be defined like this:

         $ PERL5DB="sub DB::DB {}" perl -d your-script

       Another brief debugger, slightly more useful, can be created with only the line:

         sub DB::DB {print ++$i; scalar <STDIN>}

       This debugger prints a number which increments for each statement encountered and waits
       for you to hit a newline before continuing to the next statement.

       The following debugger is actually useful:

         {
           package DB;
           sub DB  {}
           sub sub {print ++$i, " $sub\n"; &$sub}
         }

       It prints the sequence number of each subroutine call and the name of the called
       subroutine.  Note that &DB::sub is being compiled into the package "DB" through the use of
       the "package" directive.

       When it starts, the debugger reads your rc file (./.perldb or ~/.perldb under Unix), which
       can set important options.  (A subroutine (&afterinit) can be defined here as well; it is
       executed after the debugger completes its own initialization.)

       After the rc file is read, the debugger reads the PERLDB_OPTS environment variable and
       uses it to set debugger options. The contents of this variable are treated as if they were
       the argument of an "o ..." debugger command (q.v. in "Configurable Options" in perldebug).

       Debugger Internal Variables

       In addition to the file and subroutine-related variables mentioned above, the debugger
       also maintains various magical internal variables.

       ·   @DB::dbline is an alias for "@{"::_<current_file"}", which holds the lines of the
           currently-selected file (compiled by Perl), either explicitly chosen with the
           debugger's "f" command, or implicitly by flow of execution.

           Values in this array are magical in numeric context: they compare equal to zero only
           if the line is not breakable.

       ·   %DB::dbline is an alias for "%{"::_<current_file"}", which contains breakpoints and
           actions keyed by line number in the currently-selected file, either explicitly chosen
           with the debugger's "f" command, or implicitly by flow of execution.

           As previously noted, individual entries (as opposed to the whole hash) are settable.
           Perl only cares about Boolean true here, although the values used by perl5db.pl have
           the form "$break_condition\0$action".

       Debugger Customization Functions

       Some functions are provided to simplify customization.

       ·   See "Configurable Options" in perldebug for a description of options parsed by
           "DB::parse_options(string)".

       ·   "DB::dump_trace(skip[,count])" skips the specified number of frames and returns a list
           containing information about the calling frames (all of them, if "count" is missing).
           Each entry is reference to a hash with keys "context" (either ".", "$", or "@"), "sub"
           (subroutine name, or info about "eval"), "args" ("undef" or a reference to an array),
           "file", and "line".

       ·   "DB::print_trace(FH, skip[, count[, short]])" prints formatted info about caller
           frames.  The last two functions may be convenient as arguments to "<", "<<" commands.

       Note that any variables and functions that are not documented in this manpages (or in
       perldebug) are considered for internal use only, and as such are subject to change without
       notice.

Frame Listing Output Examples

       The "frame" option can be used to control the output of frame information.  For example,
       contrast this expression trace:

        $ perl -de 42
        Stack dump during die enabled outside of evals.

        Loading DB routines from perl5db.pl patch level 0.94
        Emacs support available.

        Enter h or 'h h' for help.

        main::(-e:1):   0
          DB<1> sub foo { 14 }

          DB<2> sub bar { 3 }

          DB<3> t print foo() * bar()
        main::((eval 172):3):   print foo() + bar();
        main::foo((eval 168):2):
        main::bar((eval 170):2):
        42

       with this one, once the "o"ption "frame=2" has been set:

          DB<4> o f=2
                       frame = '2'
          DB<5> t print foo() * bar()
        3:      foo() * bar()
        entering main::foo
         2:     sub foo { 14 };
        exited main::foo
        entering main::bar
         2:     sub bar { 3 };
        exited main::bar
        42

       By way of demonstration, we present below a laborious listing resulting from setting your
       "PERLDB_OPTS" environment variable to the value "f=n N", and running perl -d -V from the
       command line.  Examples using various values of "n" are shown to give you a feel for the
       difference between settings.  Long though it may be, this is not a complete listing, but
       only excerpts.

       1.
             entering main::BEGIN
              entering Config::BEGIN
               Package lib/Exporter.pm.
               Package lib/Carp.pm.
              Package lib/Config.pm.
              entering Config::TIEHASH
              entering Exporter::import
               entering Exporter::export
             entering Config::myconfig
              entering Config::FETCH
              entering Config::FETCH
              entering Config::FETCH
              entering Config::FETCH

       2.
             entering main::BEGIN
              entering Config::BEGIN
               Package lib/Exporter.pm.
               Package lib/Carp.pm.
              exited Config::BEGIN
              Package lib/Config.pm.
              entering Config::TIEHASH
              exited Config::TIEHASH
              entering Exporter::import
               entering Exporter::export
               exited Exporter::export
              exited Exporter::import
             exited main::BEGIN
             entering Config::myconfig
              entering Config::FETCH
              exited Config::FETCH
              entering Config::FETCH
              exited Config::FETCH
              entering Config::FETCH

       3.
             in  $=main::BEGIN() from /dev/null:0
              in  $=Config::BEGIN() from lib/Config.pm:2
               Package lib/Exporter.pm.
               Package lib/Carp.pm.
              Package lib/Config.pm.
              in  $=Config::TIEHASH('Config') from lib/Config.pm:644
              in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
               in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
             in  @=Config::myconfig() from /dev/null:0
              in  $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
              in  $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
              in  $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
              in  $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574
              in  $=Config::FETCH(ref(Config), 'osname') from lib/Config.pm:574
              in  $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574

       4.
             in  $=main::BEGIN() from /dev/null:0
              in  $=Config::BEGIN() from lib/Config.pm:2
               Package lib/Exporter.pm.
               Package lib/Carp.pm.
              out $=Config::BEGIN() from lib/Config.pm:0
              Package lib/Config.pm.
              in  $=Config::TIEHASH('Config') from lib/Config.pm:644
              out $=Config::TIEHASH('Config') from lib/Config.pm:644
              in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
               in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
               out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
              out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
             out $=main::BEGIN() from /dev/null:0
             in  @=Config::myconfig() from /dev/null:0
              in  $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
              out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
              in  $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
              out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
              in  $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
              out $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
              in  $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574

       5.
             in  $=main::BEGIN() from /dev/null:0
              in  $=Config::BEGIN() from lib/Config.pm:2
               Package lib/Exporter.pm.
               Package lib/Carp.pm.
              out $=Config::BEGIN() from lib/Config.pm:0
              Package lib/Config.pm.
              in  $=Config::TIEHASH('Config') from lib/Config.pm:644
              out $=Config::TIEHASH('Config') from lib/Config.pm:644
              in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
               in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
               out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
              out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
             out $=main::BEGIN() from /dev/null:0
             in  @=Config::myconfig() from /dev/null:0
              in  $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
              out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
              in  $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
              out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574

       6.
             in  $=CODE(0x15eca4)() from /dev/null:0
              in  $=CODE(0x182528)() from lib/Config.pm:2
               Package lib/Exporter.pm.
              out $=CODE(0x182528)() from lib/Config.pm:0
              scalar context return from CODE(0x182528): undef
              Package lib/Config.pm.
              in  $=Config::TIEHASH('Config') from lib/Config.pm:628
              out $=Config::TIEHASH('Config') from lib/Config.pm:628
              scalar context return from Config::TIEHASH:   empty hash
              in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
               in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
               out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
               scalar context return from Exporter::export: ''
              out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
              scalar context return from Exporter::import: ''

       In all cases shown above, the line indentation shows the call tree.  If bit 2 of "frame"
       is set, a line is printed on exit from a subroutine as well.  If bit 4 is set, the
       arguments are printed along with the caller info.  If bit 8 is set, the arguments are
       printed even if they are tied or references.  If bit 16 is set, the return value is
       printed, too.

       When a package is compiled, a line like this

           Package lib/Carp.pm.

       is printed with proper indentation.

Debugging Regular Expressions

       There are two ways to enable debugging output for regular expressions.

       If your perl is compiled with "-DDEBUGGING", you may use the -Dr flag on the command line.

       Otherwise, one can "use re 'debug'", which has effects at compile time and run time.
       Since Perl 5.9.5, this pragma is lexically scoped.

   Compile-time Output
       The debugging output at compile time looks like this:

         Compiling REx '[bc]d(ef*g)+h[ij]k$'
         size 45 Got 364 bytes for offset annotations.
         first at 1
         rarest char g at 0
         rarest char d at 0
            1: ANYOF[bc](12)
           12: EXACT <d>(14)
           14: CURLYX[0] {1,32767}(28)
           16:   OPEN1(18)
           18:     EXACT <e>(20)
           20:     STAR(23)
           21:       EXACT <f>(0)
           23:     EXACT <g>(25)
           25:   CLOSE1(27)
           27:   WHILEM[1/1](0)
           28: NOTHING(29)
           29: EXACT <h>(31)
           31: ANYOF[ij](42)
           42: EXACT <k>(44)
           44: EOL(45)
           45: END(0)
         anchored 'de' at 1 floating 'gh' at 3..2147483647 (checking floating)
               stclass 'ANYOF[bc]' minlen 7
         Offsets: [45]
               1[4] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 5[1]
               0[0] 12[1] 0[0] 6[1] 0[0] 7[1] 0[0] 9[1] 8[1] 0[0] 10[1] 0[0]
               11[1] 0[0] 12[0] 12[0] 13[1] 0[0] 14[4] 0[0] 0[0] 0[0] 0[0]
               0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 18[1] 0[0] 19[1] 20[0]
         Omitting $` $& $' support.

       The first line shows the pre-compiled form of the regex.  The second shows the size of the
       compiled form (in arbitrary units, usually 4-byte words) and the total number of bytes
       allocated for the offset/length table, usually 4+"size"*8.  The next line shows the label
       id of the first node that does a match.

       The

         anchored 'de' at 1 floating 'gh' at 3..2147483647 (checking floating)
               stclass 'ANYOF[bc]' minlen 7

       line (split into two lines above) contains optimizer information.  In the example shown,
       the optimizer found that the match should contain a substring "de" at offset 1, plus
       substring "gh" at some offset between 3 and infinity.  Moreover, when checking for these
       substrings (to abandon impossible matches quickly), Perl will check for the substring "gh"
       before checking for the substring "de".  The optimizer may also use the knowledge that the
       match starts (at the "first" id) with a character class, and no string shorter than 7
       characters can possibly match.

       The fields of interest which may appear in this line are

       "anchored" STRING "at" POS
       "floating" STRING "at" POS1..POS2
           See above.

       "matching floating/anchored"
           Which substring to check first.

       "minlen"
           The minimal length of the match.

       "stclass" TYPE
           Type of first matching node.

       "noscan"
           Don't scan for the found substrings.

       "isall"
           Means that the optimizer information is all that the regular expression contains, and
           thus one does not need to enter the regex engine at all.

       "GPOS"
           Set if the pattern contains "\G".

       "plus"
           Set if the pattern starts with a repeated char (as in "x+y").

       "implicit"
           Set if the pattern starts with ".*".

       "with eval"
           Set if the pattern contain eval-groups, such as "(?{ code })" and "(??{ code })".

       "anchored(TYPE)"
           If the pattern may match only at a handful of places, with "TYPE" being "BOL", "MBOL",
           or "GPOS".  See the table below.

       If a substring is known to match at end-of-line only, it may be followed by "$", as in
       "floating 'k'$".

       The optimizer-specific information is used to avoid entering (a slow) regex engine on
       strings that will not definitely match.  If the "isall" flag is set, a call to the regex
       engine may be avoided even when the optimizer found an appropriate place for the match.

       Above the optimizer section is the list of nodes of the compiled form of the regex.  Each
       line has format

       "   "id: TYPE OPTIONAL-INFO (next-id)

   Types of Nodes
       Here are the possible types, with short descriptions:

        # TYPE arg-description [num-args] [longjump-len] DESCRIPTION

        # Exit points

        END                no         End of program.
        SUCCEED            no         Return from a subroutine, basically.

        # Anchors:

        BOL                no         Match "" at beginning of line.
        MBOL               no         Same, assuming multiline.
        SBOL               no         Same, assuming singleline.
        EOS                no         Match "" at end of string.
        EOL                no         Match "" at end of line.
        MEOL               no         Same, assuming multiline.
        SEOL               no         Same, assuming singleline.
        BOUND              no         Match "" at any word boundary using
                                      native charset semantics for non-utf8
        BOUNDL             no         Match "" at any locale word boundary
        BOUNDU             no         Match "" at any word boundary using
                                      Unicode semantics
        BOUNDA             no         Match "" at any word boundary using ASCII
                                      semantics
        NBOUND             no         Match "" at any word non-boundary using
                                      native charset semantics for non-utf8
        NBOUNDL            no         Match "" at any locale word non-boundary
        NBOUNDU            no         Match "" at any word non-boundary using
                                      Unicode semantics
        NBOUNDA            no         Match "" at any word non-boundary using
                                      ASCII semantics
        GPOS               no         Matches where last m//g left off.

        # [Special] alternatives:

        REG_ANY            no         Match any one character (except newline).
        SANY               no         Match any one character.
        CANY               no         Match any one byte.
        ANYOF              sv         Match character in (or not in) this
                                      class, single char match only
        ANYOF_WARN_SUPER   sv         Match character in (or not in) this
                                      class, warn (if enabled) upon matching a
                                      char above Unicode max;
        ANYOF_SYNTHETIC    sv         Synthetic start class

        POSIXD             none       Some [[:class:]] under /d; the FLAGS
                                      field gives which one
        POSIXL             none       Some [[:class:]] under /l; the FLAGS
                                      field gives which one
        POSIXU             none       Some [[:class:]] under /u; the FLAGS
                                      field gives which one
        POSIXA             none       Some [[:class:]] under /a; the FLAGS
                                      field gives which one
        NPOSIXD            none       complement of POSIXD, [[:^class:]]
        NPOSIXL            none       complement of POSIXL, [[:^class:]]
        NPOSIXU            none       complement of POSIXU, [[:^class:]]
        NPOSIXA            none       complement of POSIXA, [[:^class:]]

        CLUMP              no         Match any extended grapheme cluster
                                      sequence

        # Alternation

        # BRANCH        The set of branches constituting a single choice are
        #               hooked together with their "next" pointers, since
        #               precedence prevents anything being concatenated to
        #               any individual branch.  The "next" pointer of the last
        #               BRANCH in a choice points to the thing following the
        #               whole choice.  This is also where the final "next"
        #               pointer of each individual branch points; each branch
        #               starts with the operand node of a BRANCH node.
        #
        BRANCH             node       Match this alternative, or the next...

        # Back pointer

        # BACK          Normal "next" pointers all implicitly point forward;
        #               BACK exists to make loop structures possible.
        # not used
        BACK               no         Match "", "next" ptr points backward.

        # Literals

        EXACT              str        Match this string (preceded by length).
        EXACTF             str        Match this non-UTF-8 string (not
                                      guaranteed to be folded) using /id rules
                                      (w/len).
        EXACTFL            str        Match this string (not guaranteed to be
                                      folded) using /il rules (w/len).
        EXACTFU            str        Match this string (folded iff in UTF-8,
                                      length in folding doesn't change if not
                                      in UTF-8) using /iu rules (w/len).
        EXACTFA            str        Match this string (not guaranteed to be
                                      folded) using /iaa rules (w/len).
        EXACTFU_SS         str        Match this string (folded iff in UTF-8,
                                      length in folding may change even if not
                                      in UTF-8) using /iu rules (w/len).
        EXACTFU_TRICKYFOLD str        Match this folded UTF-8 string using /iu
                                      rules

        # Do nothing types

        NOTHING            no         Match empty string.
        # A variant of above which delimits a group, thus stops optimizations
        TAIL               no         Match empty string. Can jump here from
                                      outside.

        # Loops

        # STAR,PLUS    '?', and complex '*' and '+', are implemented as
        #               circular BRANCH structures using BACK.  Simple cases
        #               (one character per match) are implemented with STAR
        #               and PLUS for speed and to minimize recursive plunges.
        #
        STAR               node       Match this (simple) thing 0 or more
                                      times.
        PLUS               node       Match this (simple) thing 1 or more
                                      times.

        CURLY              sv 2       Match this simple thing {n,m} times.
        CURLYN             no 2       Capture next-after-this simple thing
        CURLYM             no 2       Capture this medium-complex thing {n,m}
                                      times.
        CURLYX             sv 2       Match this complex thing {n,m} times.

        # This terminator creates a loop structure for CURLYX
        WHILEM             no         Do curly processing and see if rest
                                      matches.

        # Buffer related

        # OPEN,CLOSE,GROUPP     ...are numbered at compile time.
        OPEN               num 1      Mark this point in input as start of #n.
        CLOSE              num 1      Analogous to OPEN.

        REF                num 1      Match some already matched string
        REFF               num 1      Match already matched string, folded
                                      using native charset semantics for non-
                                      utf8
        REFFL              num 1      Match already matched string, folded in
                                      loc.
        REFFU              num 1      Match already matched string, folded
                                      using unicode semantics for non-utf8
        REFFA              num 1      Match already matched string, folded
                                      using unicode semantics for non-utf8, no
                                      mixing ASCII, non-ASCII

        # Named references.  Code in regcomp.c assumes that these all are after
        # the numbered references
        NREF               no-sv 1    Match some already matched string
        NREFF              no-sv 1    Match already matched string, folded
                                      using native charset semantics for non-
                                      utf8
        NREFFL             no-sv 1    Match already matched string, folded in
                                      loc.
        NREFFU             num 1      Match already matched string, folded
                                      using unicode semantics for non-utf8
        NREFFA             num 1      Match already matched string, folded
                                      using unicode semantics for non-utf8, no
                                      mixing ASCII, non-ASCII

        IFMATCH            off 1 2    Succeeds if the following matches.
        UNLESSM            off 1 2    Fails if the following matches.
        SUSPEND            off 1 1    "Independent" sub-RE.
        IFTHEN             off 1 1    Switch, should be preceded by switcher.
        GROUPP             num 1      Whether the group matched.

        # Support for long RE

        LONGJMP            off 1 1    Jump far away.
        BRANCHJ            off 1 1    BRANCH with long offset.

        # The heavy worker

        EVAL               evl 1      Execute some Perl code.

        # Modifiers

        MINMOD             no         Next operator is not greedy.
        LOGICAL            no         Next opcode should set the flag only.

        # This is not used yet
        RENUM              off 1 1    Group with independently numbered parens.

        # Trie Related

        # Behave the same as A|LIST|OF|WORDS would. The '..C' variants
        # have inline charclass data (ascii only), the 'C' store it in the
        # structure.

        TRIE               trie 1     Match many EXACT(F[ALU]?)? at once.
                                      flags==type
        TRIEC              trie       Same as TRIE, but with embedded charclass
                           charclass  data

        AHOCORASICK        trie 1     Aho Corasick stclass. flags==type
        AHOCORASICKC       trie       Same as AHOCORASICK, but with embedded
                           charclass  charclass data

        # Regex Subroutines
        GOSUB              num/ofs 2L recurse to paren arg1 at (signed) ofs
                                      arg2
        GOSTART            no         recurse to start of pattern

        # Special conditionals
        NGROUPP            no-sv 1    Whether the group matched.
        INSUBP             num 1      Whether we are in a specific recurse.
        DEFINEP            none 1     Never execute directly.

        # Backtracking Verbs
        ENDLIKE            none       Used only for the type field of verbs
        OPFAIL             none       Same as (?!)
        ACCEPT             parno 1    Accepts the current matched string.

        # Verbs With Arguments
        VERB               no-sv 1    Used only for the type field of verbs
        PRUNE              no-sv 1    Pattern fails at this startpoint if no-
                                      backtracking through this
        MARKPOINT          no-sv 1    Push the current location for rollback by
                                      cut.
        SKIP               no-sv 1    On failure skip forward (to the mark)
                                      before retrying
        COMMIT             no-sv 1    Pattern fails outright if backtracking
                                      through this
        CUTGROUP           no-sv 1    On failure go to the next alternation in
                                      the group

        # Control what to keep in $&.
        KEEPS              no         $& begins here.

        # New charclass like patterns
        LNBREAK            none       generic newline pattern

        # SPECIAL  REGOPS

        # This is not really a node, but an optimized away piece of a "long"
        # node.  To simplify debugging output, we mark it as if it were a node
        OPTIMIZED          off        Placeholder for dump.

        # Special opcode with the property that no opcode in a compiled program
        # will ever be of this type. Thus it can be used as a flag value that
        # no other opcode has been seen. END is used similarly, in that an END
        # node cant be optimized. So END implies "unoptimizable" and PSEUDO
        # mean "not seen anything to optimize yet".
        PSEUDO             off        Pseudo opcode for internal use.

       Following the optimizer information is a dump of the offset/length table, here split
       across several lines:

         Offsets: [45]
               1[4] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 5[1]
               0[0] 12[1] 0[0] 6[1] 0[0] 7[1] 0[0] 9[1] 8[1] 0[0] 10[1] 0[0]
               11[1] 0[0] 12[0] 12[0] 13[1] 0[0] 14[4] 0[0] 0[0] 0[0] 0[0]
               0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 18[1] 0[0] 19[1] 20[0]

       The first line here indicates that the offset/length table contains 45 entries.  Each
       entry is a pair of integers, denoted by "offset[length]".  Entries are numbered starting
       with 1, so entry #1 here is "1[4]" and entry #12 is "5[1]".  "1[4]" indicates that the
       node labeled "1:" (the "1: ANYOF[bc]") begins at character position 1 in the pre-compiled
       form of the regex, and has a length of 4 characters.  "5[1]" in position 12 indicates that
       the node labeled "12:" (the "12: EXACT <d>") begins at character position 5 in the pre-
       compiled form of the regex, and has a length of 1 character.  "12[1]" in position 14
       indicates that the node labeled "14:" (the "14: CURLYX[0] {1,32767}") begins at character
       position 12 in the pre-compiled form of the regex, and has a length of 1 character---that
       is, it corresponds to the "+" symbol in the precompiled regex.

       "0[0]" items indicate that there is no corresponding node.

   Run-time Output
       First of all, when doing a match, one may get no run-time output even if debugging is
       enabled.  This means that the regex engine was never entered and that all of the job was
       therefore done by the optimizer.

       If the regex engine was entered, the output may look like this:

         Matching '[bc]d(ef*g)+h[ij]k$' against 'abcdefg__gh__'
           Setting an EVAL scope, savestack=3
            2 <ab> <cdefg__gh_>    |  1: ANYOF
            3 <abc> <defg__gh_>    | 11: EXACT <d>
            4 <abcd> <efg__gh_>    | 13: CURLYX {1,32767}
            4 <abcd> <efg__gh_>    | 26:   WHILEM
                                       0 out of 1..32767  cc=effff31c
            4 <abcd> <efg__gh_>    | 15:     OPEN1
            4 <abcd> <efg__gh_>    | 17:     EXACT <e>
            5 <abcde> <fg__gh_>    | 19:     STAR
                                    EXACT <f> can match 1 times out of 32767...
           Setting an EVAL scope, savestack=3
            6 <bcdef> <g__gh__>    | 22:       EXACT <g>
            7 <bcdefg> <__gh__>    | 24:       CLOSE1
            7 <bcdefg> <__gh__>    | 26:       WHILEM
                                           1 out of 1..32767  cc=effff31c
           Setting an EVAL scope, savestack=12
            7 <bcdefg> <__gh__>    | 15:         OPEN1
            7 <bcdefg> <__gh__>    | 17:         EXACT <e>
              restoring \1 to 4(4)..7
                                           failed, try continuation...
            7 <bcdefg> <__gh__>    | 27:         NOTHING
            7 <bcdefg> <__gh__>    | 28:         EXACT <h>
                                           failed...
                                       failed...

       The most significant information in the output is about the particular node of the
       compiled regex that is currently being tested against the target string.  The format of
       these lines is

       "    "STRING-OFFSET <PRE-STRING> <POST-STRING>   |ID:  TYPE

       The TYPE info is indented with respect to the backtracking level.  Other incidental
       information appears interspersed within.

Debugging Perl Memory Usage

       Perl is a profligate wastrel when it comes to memory use.  There is a saying that to
       estimate memory usage of Perl, assume a reasonable algorithm for memory allocation,
       multiply that estimate by 10, and while you still may miss the mark, at least you won't be
       quite so astonished.  This is not absolutely true, but may provide a good grasp of what
       happens.

       Assume that an integer cannot take less than 20 bytes of memory, a float cannot take less
       than 24 bytes, a string cannot take less than 32 bytes (all these examples assume 32-bit
       architectures, the result are quite a bit worse on 64-bit architectures).  If a variable
       is accessed in two of three different ways (which require an integer, a float, or a
       string), the memory footprint may increase yet another 20 bytes.  A sloppy malloc(3)
       implementation can inflate these numbers dramatically.

       On the opposite end of the scale, a declaration like

         sub foo;

       may take up to 500 bytes of memory, depending on which release of Perl you're running.

       Anecdotal estimates of source-to-compiled code bloat suggest an eightfold increase.  This
       means that the compiled form of reasonable (normally commented, properly indented etc.)
       code will take about eight times more space in memory than the code took on disk.

       The -DL command-line switch is obsolete since circa Perl 5.6.0 (it was available only if
       Perl was built with "-DDEBUGGING").  The switch was used to track Perl's memory
       allocations and possible memory leaks.  These days the use of malloc debugging tools like
       Purify or valgrind is suggested instead.  See also "PERL_MEM_LOG" in perlhacktips.

       One way to find out how much memory is being used by Perl data structures is to install
       the Devel::Size module from CPAN: it gives you the minimum number of bytes required to
       store a particular data structure.  Please be mindful of the difference between the size()
       and total_size().

       If Perl has been compiled using Perl's malloc you can analyze Perl memory usage by setting
       $ENV{PERL_DEBUG_MSTATS}.

   Using $ENV{PERL_DEBUG_MSTATS}
       If your perl is using Perl's malloc() and was compiled with the necessary switches (this
       is the default), then it will print memory usage statistics after compiling your code when
       "$ENV{PERL_DEBUG_MSTATS} > 1", and before termination of the program when
       "$ENV{PERL_DEBUG_MSTATS} >= 1".  The report format is similar to the following example:

         $ PERL_DEBUG_MSTATS=2 perl -e "require Carp"
         Memory allocation statistics after compilation: (buckets 4(4)..8188(8192)
            14216 free:   130   117    28     7     9   0   2     2   1 0 0
                       437    61    36     0     5
            60924 used:   125   137   161    55     7   8   6    16   2 0 1
                        74   109   304    84    20
         Total sbrk(): 77824/21:119. Odd ends: pad+heads+chain+tail: 0+636+0+2048.
         Memory allocation statistics after execution:   (buckets 4(4)..8188(8192)
            30888 free:   245    78    85    13     6   2   1     3   2 0 1
                       315   162    39    42    11
           175816 used:   265   176  1112   111    26  22  11    27   2 1 1
                       196   178  1066   798    39
         Total sbrk(): 215040/47:145. Odd ends: pad+heads+chain+tail: 0+2192+0+6144.

       It is possible to ask for such a statistic at arbitrary points in your execution using the
       mstat() function out of the standard Devel::Peek module.

       Here is some explanation of that format:

       "buckets SMALLEST(APPROX)..GREATEST(APPROX)"
           Perl's malloc() uses bucketed allocations.  Every request is rounded up to the closest
           bucket size available, and a bucket is taken from the pool of buckets of that size.

           The line above describes the limits of buckets currently in use.  Each bucket has two
           sizes: memory footprint and the maximal size of user data that can fit into this
           bucket.  Suppose in the above example that the smallest bucket were size 4.  The
           biggest bucket would have usable size 8188, and the memory footprint would be 8192.

           In a Perl built for debugging, some buckets may have negative usable size.  This means
           that these buckets cannot (and will not) be used.  For larger buckets, the memory
           footprint may be one page greater than a power of 2.  If so, the corresponding power
           of two is printed in the "APPROX" field above.

       Free/Used
           The 1 or 2 rows of numbers following that correspond to the number of buckets of each
           size between "SMALLEST" and "GREATEST".  In the first row, the sizes (memory
           footprints) of buckets are powers of two--or possibly one page greater.  In the second
           row, if present, the memory footprints of the buckets are between the memory
           footprints of two buckets "above".

           For example, suppose under the previous example, the memory footprints were

                free:    8     16    32    64    128  256 512 1024 2048 4096 8192
                      4     12    24    48    80

           With a non-"DEBUGGING" perl, the buckets starting from 128 have a 4-byte overhead, and
           thus an 8192-long bucket may take up to 8188-byte allocations.

       "Total sbrk(): SBRKed/SBRKs:CONTINUOUS"
           The first two fields give the total amount of memory perl sbrk(2)ed (ess-broken? :-)
           and number of sbrk(2)s used.  The third number is what perl thinks about continuity of
           returned chunks.  So long as this number is positive, malloc() will assume that it is
           probable that sbrk(2) will provide continuous memory.

           Memory allocated by external libraries is not counted.

       "pad: 0"
           The amount of sbrk(2)ed memory needed to keep buckets aligned.

       "heads: 2192"
           Although memory overhead of bigger buckets is kept inside the bucket, for smaller
           buckets, it is kept in separate areas.  This field gives the total size of these
           areas.

       "chain: 0"
           malloc() may want to subdivide a bigger bucket into smaller buckets.  If only a part
           of the deceased bucket is left unsubdivided, the rest is kept as an element of a
           linked list.  This field gives the total size of these chunks.

       "tail: 6144"
           To minimize the number of sbrk(2)s, malloc() asks for more memory.  This field gives
           the size of the yet unused part, which is sbrk(2)ed, but never touched.

SEE ALSO

       perldebug, perlguts, perlrun re, and Devel::DProf.