perldebguts(1)


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.)

       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).

   *   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
   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 "SBOL", "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 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.
    ANYOF           sv 1       Match character in (or not in) this class,
                               single char match only
    ANYOFD          sv 1       Like ANYOF, but /d is in effect
    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
                               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...

    # Literals

    EXACT           str        Match this string (preceded by length).
    EXACTL          str        Like EXACT, but /l is in effect (used so
                               locale-related warnings can be checked for).
    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
    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 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.
                               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

    # 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          no-sv 1    Same as (?!), but with verb arg
    ACCEPT          no-sv/num  Accepts the current matched string, with
                    2L         verbar

    # 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.





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