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PERLDEBGUTS(1)			 Perl Programmers Reference Guide		   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):

       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)".  Code assertions in regexes look like "(re_eval 19)".

	   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)" or
	   "(re_eval 19)".

       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)" or "(re_eval 19)".

       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   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, or "(re_eval 19)" for those within regex code
	   assertions.

       o   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.

       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.)

       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.

       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.

	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
	ANYOFV	   sv	   Match character in (or not in) this class, can
			   match-multiple chars
	ALNUM	   no	   Match any alphanumeric character using native charset
			   semantics for non-utf8
	ALNUML	   no	   Match any alphanumeric char in locale
	ALNUMU	   no	   Match any alphanumeric char using Unicode semantics
	ALNUMA	   no	   Match [A-Za-z_0-9]
	NALNUM	   no	   Match any non-alphanumeric character using native charset
			   semantics for non-utf8
	NALNUML    no	   Match any non-alphanumeric char in locale
	NALNUMU    no	   Match any non-alphanumeric char using Unicode semantics
	NALNUMA    no	   Match [^A-Za-z_0-9]
	SPACE	   no	   Match any whitespace character using native charset
			   semantics for non-utf8
	SPACEL	   no	   Match any whitespace char in locale
	SPACEU	   no	   Match any whitespace char using Unicode semantics
	SPACEA	   no	   Match [ \t\n\f\r]
	NSPACE	   no	   Match any non-whitespace character using native charset
			   semantics for non-utf8
	NSPACEL    no	   Match any non-whitespace char in locale
	NSPACEU    no	   Match any non-whitespace char using Unicode semantics
	NSPACEA    no	   Match [^ \t\n\f\r]
	DIGIT	   no	   Match any numeric character using native charset semantics
			   for non-utf8
	DIGITL	   no	   Match any numeric character in locale
	DIGITA	   no	   Match [0-9]
	NDIGIT	   no	   Match any non-numeric character using native charset
	i		   semantics for non-utf8
	NDIGITL    no	   Match any non-numeric character in locale
	NDIGITA    no	   Match [^0-9]
	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 string, folded, native charset semantics for
			   non-utf8 (prec. by length).
	EXACTFL    str	   Match this string, folded in locale (w/len).
	EXACTFU    str	   Match this string, folded, Unicode semantics for non-utf8
			   (prec. by length).
	EXACTFA    str	   Match this string, folded, Unicode semantics for non-utf8,
			   but no ASCII-range character matches outside ASCII (prec.
			   by length),.

	# 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.
	# NOTE: the relative order of the TRIE-like regops  is significant

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

	# For start classes, contains an added fail table.
	AHOCORASICK trie 1   Aho Corasick stclass. flags==type
	AHOCORASICKC charclass Same as AHOCORASICK, but with embedded 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
	VERTWS	   none      vertical whitespace	 (Perl 6)
	NVERTWS    none      not vertical whitespace	 (Perl 6)
	HORIZWS    none      horizontal whitespace	 (Perl 6)
	NHORIZWS   none      not horizontal whitespace	 (Perl 6)

	FOLDCHAR   codepoint 1 codepoint with tricky case folding properties.

	# 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.

perl v5.16.3				    2013-03-04				   PERLDEBGUTS(1)
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