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 set to identify the called subroutine. (This doesn't happen if the calling subroutine was compiled in the "DB" package.) $DB::sub normally holds the name of the called subroutine, if it has a name by which it can be looked up. Failing that, $DB::sub will hold a reference to the called subroutine. Either way, the &DB::sub subroutine can use $DB::sub as a reference by which to call the called subroutine, which it will normally want to do.

    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 set to identify the subroutine being entered. The call to &DB::goto does not replace the "goto"; the requested subroutine will still be entered once &DB::goto has returned. $DB::sub normally holds the name of the subroutine being entered, if it has one. Failing that, $DB::sub will hold a reference to the subroutine being entered. Unlike when &DB::sub is called, it is not guaranteed that $DB::sub can be used as a reference to operate on 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, and "-Drv" for more verbose information.

Otherwise, one can "use re 'debug'", which has effects at both 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 /u 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 /u 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         Match character in (or not in) this class,
                  charclass  single char match only
 ANYOFD           sv         Like ANYOF, but /d is in effect
                  charclass
 ANYOFL           sv         Like ANYOF, but /l is in effect
                  charclass
 ANYOFPOSIXL      sv         Like ANYOFL, but matches [[:posix:]]
                  charclass_ classes
                  posixl
 ANYOFH           sv 1       Like ANYOF, but only has "High" matches,
                             none in the bitmap; non-zero flags "f"
                             means "f" is the first UTF-8 byte shared in
                             common by all code points matched
 ANYOFM           byte 1     Like ANYOF, but matches an invariant byte
                             as determined by the mask and arg
 NANYOFM          byte 1     complement of ANYOFM

 # 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 string using /id rules (w/len);
                             (string not UTF-8, not guaranteed to be
                             folded).
 EXACTFL          str        Match this string using /il rules (w/len);
                             (string not guaranteed to be folded).
 EXACTFU          str        Match this string using /iu rules (w/len);
                             (string folded iff in UTF-8; non-UTF8
                             folded length <= unfolded).
 EXACTFAA         str        Match this string using /iaa rules (w/len)
                             (string folded iff in UTF-8; non-UTF8
                             folded length <= unfolded).

 EXACTFUP         str        Match this string using /iu rules (w/len);
                             (string not UTF-8, not guaranteed to be
                             folded; and its Problematic).

 EXACTFLU8        str        Like EXACTFU, but use /il, UTF-8, folded,
                             and everything in it is above 255.
 EXACTFAA_NO_TRIE str        Match this string using /iaa rules (w/len)
                             (string not UTF-8, not guaranteed to be
                             folded, not currently trie-able).

 EXACT_ONLY8      str        Like EXACT, but only UTF-8 encoded targets
                             can match
 EXACTFU_ONLY8    str        Like EXACTFU, but only UTF-8 encoded
                             targets can match

 EXACTFU_S_EDGE   str        /di rules, but nothing in it precludes /ui,
                             except begins and/or ends with [Ss];
                             (string not UTF-8; compile-time only).

 # 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      Close corresponding OPEN of #n.
 SROPEN           none       Same as OPEN, but for script run
 SRCLOSE          none       Close preceding SROPEN

 REF              num 1      Match some already matched string
 REFF             num 1      Match already matched string, using /di
                             rules.
 REFFL            num 1      Match already matched string, using /li
                             rules.
 REFFU            num 1      Match already matched string, usng /ui.
 REFFA            num 1      Match already matched string, using /aai
                             rules.

 # 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, using /di
                             rules.
 NREFFL           no-sv 1    Match already matched string, using /li
                             rules.
 NREFFU           num 1      Match already matched string, using /ui
                             rules.
 NREFFA           num 1      Match already matched string, using /aai
                             rules.

 # 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 1    Succeeds if the following matches; non-zero
                             flags "f", next_off "o" means lookbehind
                             assertion starting "f..(f-o)" characters
                             before current
 UNLESSM          off 1 1    Fails if the following matches; non-zero
                             flags "f", next_off "o" means lookbehind
                             assertion starting "f..(f-o)" characters
                             before current
 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.