perldebguts

       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):

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

       o   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.

       o   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)".

       o   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)".

       o   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.

       o   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.

       o   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.)

           If the call is to an lvalue subroutine, and &DB::lsub is defined
           &DB::lsub(args) is called instead, otherwise falling back to
           &DB::sub(args).

       o   When execution of the program uses "goto" to enter a non-XS
           subroutine and the 0x80 bit is set in $^P, a call to &DB::goto is
           made, with $DB::sub holding the name of the subroutine being
           entered.

       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
       Debugger Internal Variables

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

       o   @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.

       o   %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.

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

       o   "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".

       o   "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.
        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
              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.
               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]

         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 "SBOL", "MBOL", or "GPOS".  See the table below.

       If a substring is known to match at end-of-line only, it may be

   Types of Nodes
       Here are the current 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.

        # Line Start Anchors:
        SBOL            no         Match "" at beginning of line: /^/, /\A/
        MBOL            no         Same, assuming multiline: /^/m

        # Line End Anchors:
        SEOL            no         Match "" at end of line: /$/
        MEOL            no         Same, assuming multiline: /$/m
        EOS             no         Match "" at end of string: /\z/

        # Match Start Anchors:
        GPOS            no         Matches where last m//g left off.

        # Word Boundary Opcodes:
        BOUND           no         Like BOUNDA for non-utf8, otherwise match ""
                                   between any Unicode \w\W or \W\w
        BOUNDL          no         Like BOUND/BOUNDU, but \w and \W are defined
                                   by current locale
        BOUNDU          no         Match "" at any boundary of a given type
                                   using Unicode rules
        BOUNDA          no         Match "" at any boundary between \w\W or
                                   \W\w, where \w is [_a-zA-Z0-9]
        NBOUND          no         Like NBOUNDA for non-utf8, otherwise match
                                   "" between any Unicode \w\w or \W\W
        NBOUNDL         no         Like NBOUND/NBOUNDU, but \w and \W are
                                   defined by current locale
        NBOUNDU         no         Match "" at any non-boundary of a given type
                                   using using Unicode rules
        NBOUNDA         no         Match "" betweeen any \w\w or \W\W, where \w
                                   is [_a-zA-Z0-9]

        # [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 1       Match character in (or not in) this class,
                                   single char match only
        ANYOFL          sv 1       Like ANYOF, but /l is in effect

        # POSIX Character Classes:
        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
        # 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...

        # Literals

        EXACT           str        Match this string (preceded by length).
        EXACTL          str        Like EXACT, but /l is in effect.
        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).
        EXACTFLU8       str        Rare cirucmstances: like EXACTFU, but is
                                   under /l, UTF-8, folded, and everything in
                                   it is above 255.
        EXACTFA_NO_TRIE str        Match this string (which is not trie-able;
                                   not guaranteed to be folded) using /iaa
                                   rules (w/len).

        # 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.  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

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

        # Support for long RE
        LONGJMP         off 1 1    Jump far away.
        BRANCHJ         off 1 1    BRANCH with long offset.

        # Special Case Regops
        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.

        # The heavy worker

        EVAL            evl/flags  Execute some Perl code.
                        2L

        # 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.
        # 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

   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
       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.

       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.

perl v5.22.1                      2018-11-19                    PERLDEBGUTS(1)
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