PERLDEBGUTS(1)

📦
version 5.42.3 · 22 versions
, in fedora 43, fedora 44

💖 NAME 💖

perldebguts(1) — Guts of Perl debugging 🫶✨

✨ DESCRIPTION ✨

okay so this is NOT perldebug, bestie. perldebug is the one that tells you how to use the debugger. This manpage? It's the low-level lore about the debugger's internals, and it ranges 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 file 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 $ENV{PERL5DB} (or BEGIN {require 'perl5db.pl'} if not present) before the first line of your program.
  • If the -dt switch is supplied, PERL5DB_THREADED=1 is set in the environment. perl5db.pl or your custom debugger can use this to enable threading support. 🧵
  • Each array @{"_<$filename"} holds the lines of $filename for a file compiled by Perl. The same is also true for evaled strings that contain subroutines, or which are currently being executed. The $filename for evaled 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 evaled 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 $filename for evaled strings looks like (eval 34).
  • After each required file is compiled, but before it is executed, DB::postponed(*{"_<$filename"}) is called if the subroutine DB::postponed exists. Here, the $filename is the expanded name of the required 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 evals.
  • 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: 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: 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 💅

Okay bestie, the frame option? It's literally the director's cut of your frame info. Watch how the vibe changes.

So first, the basic 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

And now the glow-up version, once the option frame=2 is 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

See the difference? It's giving narrative arc. ✨

By way of demonstration (and honestly, just to flex), here's a laborious listing you get 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 so you can feel the difference between settings. Long though it may be, this is not a complete listing, bestie — only excerpts. 🫶

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
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
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
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
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
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. Like, that's the whole point. 🌳

Now the frame bits, and bestie, they stack:

  • 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. Cute, right? 💖

🦋 Debugging Regular Expressions 🦋

Okay so, there are two ways to enable debugging output for regular expressions.

  1. If your perl is compiled with -DDEBUGGING, you may use the -Dr flag on the command line, and -Drv for more verbose information.
  2. 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. (Yes, it got scoped. Iconic.)

✨ 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.

Let me break this down for you, because it's a lot 💀:

  • 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.

Then 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 info is what stops us from yeeting ourselves into the (slow) regex engine on strings that are definitely not gonna match. And if the "isall" flag is set, a call to the regex engine can be dodged entirely — even when the optimizer did find an appropriate place for the match. slay 💅

Right above the optimizer section sits the list of nodes of the compiled form of the regex. Every single line is formatted like this:

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

Types of Nodes 🧬✨

okay bestie here are all the currently possible types, with short descriptions. The header line for each one goes like this:

# TYPE arg-description [regnode-struct-suffix] [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 like BOUNDU
  • 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 like BOUNDU
  • 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 /u rules.
  • NBOUNDA — no — Match "" between 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, suffix charclass — Match character in (or not in) this class, single char match only
  • ANYOFD — sv, suffix charclass — Like ANYOF, but /d is in effect
  • ANYOFL — sv, suffix charclass — Like ANYOF, but /l is in effect
  • ANYOFPOSIXL — sv, suffix charclass_posixl — Like ANYOFL, but matches [[:posix:]] classes
  • ANYOFH — sv, longjump 1 — Like ANYOF, but only has "High" matches, none in the bitmap; the flags field contains the lowest matchable UTF-8 start byte
  • ANYOFHb — sv, longjump 1 — Like ANYOFH, but all matches share the same UTF-8 start byte, given in the flags field
  • ANYOFHr — sv, longjump 1 — Like ANYOFH, but the flags field contains packed bounds for all matchable UTF-8 start bytes.
  • ANYOFHs — sv:str, longjump 1 — Like ANYOFHb, but has a string field that gives the leading matchable UTF-8 bytes; flags field is len
  • ANYOFR — packed, longjump 1 — Matches any character in the range given by its packed args: upper 12 bits is the max delta from the base lower 20; the flags field contains the lowest matchable UTF-8 start byte
  • ANYOFRb — packed, longjump 1 — Like ANYOFR, but all matches share the same UTF-8 start byte, given in the flags field
  • ANYOFHbbm — none, suffix bbm — Like ANYOFHb, but only for 2-byte UTF-8 characters; uses a bitmap to match the continuation byte
  • ANYOFM — byte, longjump 1 — Like ANYOF, but matches an invariant byte as determined by the mask and arg
  • NANYOFM — byte, longjump 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, longjump 1 — Match this alternative, or the next...

Literals 🔤

  • EXACT — str — Match this string (flags field is the length).
In a long string node, the U32 argument is the length, and is immediately followed by the string.
  • LEXACT — len:str, longjump 1 — Match this long string (preceded by length; flags unused).
  • EXACTL — str — Like EXACT, but /l is in effect (used so locale-related warnings can be checked for)
  • EXACTF — str — Like EXACT, but match using /id rules; (string not UTF-8, ASCII folded; non-ASCII not)
  • EXACTFL — str — Like EXACT, but match using /il rules; (string not likely to be folded)
  • EXACTFU — str — Like EXACT, but match using /iu rules; (string folded)
  • EXACTFAA — str — Like EXACT, but match using /iaa rules; (string folded except MICRO in non-UTF8 patterns; doesn't contain SHARP S unless UTF-8; folded length <= unfolded)
  • EXACTFAA_NO_TRIE — str — Like EXACTFAA, (string not UTF-8, folded except: MICRO, SHARP S; folded length <= unfolded, not currently trie-able)
  • EXACTFUP — str — Like EXACT, but match using /iu rules; (string not UTF-8, folded except MICRO: hence Problematic)
  • EXACTFLU8 — str — Like EXACTFU, but use /il, UTF-8, (string is folded, and everything in it is above 255
  • EXACT_REQ8 — str — Like EXACT, but only UTF-8 encoded targets can match
  • LEXACT_REQ8 — len:str, longjump 1 — Like LEXACT, but only UTF-8 encoded targets can match
  • EXACTFU_REQ8 — 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)

New charclass like patterns ✨

  • LNBREAK — none — generic newline pattern
These 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, longjump 1 — Match many EXACT(F[ALU]?)? at once. flags==type
  • TRIEC — trie, suffix charclass — Same as TRIE, but with embedded charclass data
  • AHOCORASICK — trie, longjump 1 — Aho Corasick stclass. flags==type
  • AHOCORASICKC — trie, suffix charclass — Same as AHOCORASICK, but with embedded charclass data

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: /A{0,}B/ where A is width 1 char
  • PLUS — node — Match this (simple) thing 1 or more times: /A{1,}B/ where A is width 1 char
  • CURLY — sv, longjump 3 — Match this (simple) thing {n,m} times: /A{m,n}B/ where A is width 1 char
  • CURLYN — no, longjump 3 — Capture next-after-this simple thing: /(A){m,n}B/ where A is width 1 char
  • CURLYM — no, longjump 3 — Capture this medium-complex thing {n,m} times: /(A){m,n}B/ where A is fixed-length
  • CURLYX — sv, longjump 3 — Match/Capture this complex thing {n,m} times.
This terminator creates a loop structure for CURLYX
  • WHILEM — no — Do curly processing and see if rest matches.
OPEN, CLOSE, GROUPP — ...are numbered at compile time.
  • OPEN — num, longjump 1 — Mark this point in input as start of #n.
  • CLOSE — num, longjump 1 — Close corresponding OPEN of #n.
  • SROPEN — none — Same as OPEN, but for script run
  • SRCLOSE — none — Close preceding SROPEN
  • REF — num 2 — matches some string you already matched 💖
  • REFF — num 2 — matches a string you already matched, but doing it with the /di rules applied ✨
  • REFFL — num 2 — matches a string you already matched, applying the /li rules 🌸
  • REFFU — num 2 — matches a string you already matched, applying the /ui rules 🦋
  • REFFA — num 2 — matches a string you already matched, applying the /aai rules 💅
# Named references. Code in regcomp.c assumes that these all are after the numbered references
  • REFN — no-sv 2 — matches some string you already matched 🎀
  • REFFN — no-sv 2 — matches a string you already matched, with the /di rules
  • REFFLN — no-sv 2 — matches a string you already matched, with the /li rules
  • REFFUN — num 2 — matches a string you already matched, with the /ui rules
  • REFFAN — num 2 — matches a string you already matched, with the /aai rules
# Support for long RE
  • LONGJMP — off 1 1 — jumps far away, no cap 🏃‍♀️💨
  • BRANCHJ — off 2 1 — a BRANCH that has a long offset
# Special Case Regops
  • IFMATCH — off 1 1 — succeeds if whatever comes next matches. Non-zero flags "f", next_off "o" means it's a lookbehind assertion starting "f..(f-o)" characters before the current spot 👀
  • UNLESSM — off 1 1 — fails if whatever comes next matches. Same deal: non-zero flags "f", next_off "o" means a lookbehind assertion starting "f..(f-o)" characters before the current spot 🙈
  • SUSPEND — off 1 1 — an "Independent" sub-RE, living its own life 🦋
  • IFTHEN — off 1 1 — a switch, and it should be preceded by a switcher
  • GROUPP — num 1 — whether the group matched
# The heavy worker
  • EVAL — evl/flags 2 — executes some Perl code. the moment the regex calls in the big guns 🔥
# Modifiers
  • MINMOD — no — the next operator is not greedy
  • LOGICAL — no — the next opcode should only set the flag
# This is not used yet
  • RENUM — off 1 1 — a group whose parens are numbered independently 👑
# Regex Subroutines
  • GOSUB — num/ofs 2 — recurses to paren arg1 at the (signed) ofs arg2
# Special conditionals
  • GROUPPN — no-sv 1 — whether the group matched
  • INSUBP — num 1 — whether we are inside a specific recurse
  • DEFINEP — none 1 — never executes directly, it's just not that kind of op 💅
# Backtracking Verbs
  • ENDLIKE — none — only ever used for the type field of verbs
  • OPFAIL — no-sv 1 — same as (?!, but with a verb arg
  • ACCEPT — no-sv/num 2 — accepts the current matched string, with verbar
# Verbs With Arguments
  • VERB — no-sv 1 — only ever used for the type field of verbs
  • PRUNE — no-sv 1 — the pattern fails at this startpoint if there's no backtracking through this
  • MARKPOINT — no-sv 1 — pushes the current location so cut can roll it back
  • SKIP — no-sv 1 — on failure, skip forward (to the mark) before retrying
  • COMMIT — no-sv 1 — the pattern fails outright if backtracking goes through this 💀
  • CUTGROUP — no-sv 1 — on failure, go to the next alternation in the group
# Control what to keep in $&.
  • KEEPS — no — $& begins right here
# Validate that lookbehind IFMATCH and UNLESSM end at the right place
  • LOOKBEHIND_END — no — returns from lookbehind (IFMATCH/UNLESSM) and validates the position
# 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 — a placeholder for the 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 can't be optimized. So END implies "unoptimizable" and PSEUDO mean "not seen anything to optimize yet".
  • PSEUDO — off — a pseudo opcode for internal use only
  • REGEX_SET — depth p — a regex set, a temporary node used in pre-optimization compilation ✨

Okay so next up: after all the optimizer info, you get a dump of the offset/length table, and it's split across a bunch of lines like this 💖

 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]

Let's decode this together bestie, because at first glance it's giving nothing 😭

That very first line is telling you the offset/length table has 45 entries. Every single entry is a pair of integers, written like "offset[length]". Entries are numbered starting at 1, so entry #1 here is "1[4]" and entry #12 is "5[1]".

  • "1[4]" says the node labeled "1:" (which is the "1: ANYOF[bc]") starts at character position 1 in the pre-compiled form of the regex, and is 4 characters long.
  • "5[1]" sitting in position 12 says the node labeled "12:" (the "12: EXACT <d>") starts at character position 5 in the pre-compiled form of the regex, and is 1 character long.
  • "12[1]" sitting in position 14 says the node labeled "14:" (the "14: CURLYX[0] {1,32767}") starts at character position 12 in the pre-compiled form of the regex, and is 1 character long — meaning it corresponds to the "+" symbol in the precompiled regex. 🎀

And any "0[0]" items? Those are just saying there's no corresponding node there, periodt. 💅

Run-time Output

First things first, bestie: when you're doing a match, you might get no run-time output at all even if debugging is turned on 😭. That's not a bug — it means the regex engine was never even entered, and the whole entire job got handled by the optimizer. Iconic behavior honestly. ✨

But if the regex engine was entered, the output can 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 info in that whole wall of text is about the specific node of the compiled regex that's currently being tested against the target string. The format of those lines is:

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

And the TYPE info is indented depending on the backtracking level. Other incidental info just kinda shows up interspersed within. 🦋

🧠 Debugging Perl Memory Usage 🧠

Okay bestie, real talk: Perl is a profligate wastrel when it comes to memory use 💸 and that's just the vibe. There's a saying that to estimate memory usage of Perl, you 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. It's not absolutely true, but it 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, and the results 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. And 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. not me crying over an empty sub 💀

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. it's giving ✨expansion✨

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(). 👑 (they are NOT the same thing, iykyk)

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, because bestie that output is a lot:

"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. ✨ untouched, like my to-do list ✨

👀 SEE ALSO 👀

perldebug, perl5db.pl, perlguts, perlrun, re, and Devel::DProf. go read them fr fr 📚💖

and that's a wrap on this whole page, bestie!! 🎀 you made it to the end of PERLDEBGUTS(1) — this was perl v5.42.3, dated 2026-08-03, and you're officially a debugger-internals girlie now 👑🦋 slay, ily, go debug something iconic 💅

📦 where this page comes from