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 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 $ENV{PERL5DB} (or "BEGIN {require
'perl5db.pl'}" if not present) before the first line of your
program.
• Each array "@{"_<$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'$".
The optimizer-specific information is used to avoid entering (a slow)
regex engine on strings that will not definitely match. If the "isall"
flag is set, a call to the regex engine may be avoided even when the
optimizer found an appropriate place for the match.
Above the optimizer section is the list of nodes of the compiled form of
the regex. Each line has format
" "id: TYPE OPTIONAL-INFO (next-id)
Types of Nodes
Here are the current possible types, with short descriptions:
# TYPE arg-description [regnode-struct-suffix] [longjump-len] DESCRIPTION
# Exit points
END no End of program.
SUCCEED no Return from a subroutine, basically.
# Line Start Anchors:
SBOL no Match "" at beginning of line: /^/, /\A/
MBOL no Same, assuming multiline: /^/m
# Line End Anchors:
SEOL no Match "" at end of line: /$/
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 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; the flags field
contains the lowest matchable UTF-8 start
byte
ANYOFHb sv 1 Like ANYOFH, but all matches share the same
UTF-8 start byte, given in the flags field
ANYOFHr sv 1 Like ANYOFH, but the flags field contains
packed bounds for all matchable UTF-8 start
bytes.
ANYOFHs sv:str 1 Like ANYOFHb, but has a string field that
gives the leading matchable UTF-8 bytes;
flags field is len
ANYOFR packed 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 1 Like ANYOFR, but all matches share the same
UTF-8 start byte, given in the flags field
ANYOFHbbm none bbm Like ANYOFHb, but only for 2-byte UTF-8
characters; uses a bitmap to match the
continuation byte
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 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 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 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
# 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
# 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 3 Match this (simple) thing {n,m} times:
/A{m,n}B/ where A is width 1 char
CURLYN no 3 Capture next-after-this simple thing:
/(A){m,n}B/ where A is width 1 char
CURLYM no 3 Capture this medium-complex thing {n,m}
times: /(A){m,n}B/ where A is fixed-length
CURLYX sv 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.
# 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 2 Match some already matched string
REFF num 2 Match already matched string, using /di
rules.
REFFL num 2 Match already matched string, using /li
rules.
REFFU num 2 Match already matched string, usng /ui.
REFFA num 2 Match already matched string, using /aai
rules.
# Named references. Code in regcomp.c assumes that these all are after
# the numbered references
REFN no-sv 2 Match some already matched string
REFFN no-sv 2 Match already matched string, using /di
rules.
REFFLN no-sv 2 Match already matched string, using /li
rules.
REFFUN num 2 Match already matched string, using /ui
rules.
REFFAN num 2 Match already matched string, using /aai
rules.
# Support for long RE
LONGJMP off 1 1 Jump far away.
BRANCHJ off 2 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.
2
# 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.
# Regex Subroutines
GOSUB num/ofs 2 recurse to paren arg1 at (signed) ofs arg2
# Special conditionals
GROUPPN 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
2 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.
# Validate that lookbehind IFMATCH and UNLESSM end at the right place
LOOKBEHIND_END no Return from lookbehind (IFMATCH/UNLESSM)
and validate 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 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.
REGEX_SET depth p Regex set, temporary node used in pre-
optimization compilation
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, perl5db.pl, perlguts, perlrun, re, and Devel::DProf.
perl v5.40.1 2026-08-30 PERLDEBGUTS(1)
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