--Maurice Wilkes, 1949
If you invoke Perl with the -d switch, your script runs under the Perl source debugger. This works like an interactive Perl environment, prompting for debugger commands that let you examine source code, set breakpoints, get stack backtraces, change the values of variables, etc. This is so convenient that you often fire up the debugger all by itself just to test out Perl constructs interactively to see what they do. For example:
perl -d -e 42In Perl, the debugger is not a separate program as it usually is in the typical compiled environment. Instead, the -d flag tells the compiler to insert source information into the parse trees it's about to hand off to the interpreter. That means your code must first compile correctly for the debugger to work on it. Then when the interpreter starts up, it preloads a Perl library file containing the debugger itself.
The program will halt right before the first run-time executable statement (but see below regarding compile-time statements) and ask you to enter a debugger command. Contrary to popular expectations, whenever the debugger halts and shows you a line of code, it always displays the line it's about to execute, rather than the one it has just executed.
Any command not recognized by the debugger is directly executed (eval'd) as Perl code in the current package. (The debugger uses the DB package for its own state information.)
Leading white space before a command would cause the debugger to think it's NOT a debugger command but for Perl, so be careful not to do that.
If you supply another debugger command as an argument to the h command, it prints out the description for just that command. The special argument of h h produces a more compact help listing, designed to fit together on one screen.
If the output of the h command (or any command, for that matter) scrolls past your screen, either precede the command with a leading pipe symbol so it's run through your pager, as in
DB> |hYou may change the pager which is used via O pager=... command.
The DB::OUT filehandle is opened to /dev/tty, regardless of
where STDOUT may be redirected to.
The details of printout are governed by multiple Options.
V DB filename lineUse ~pattern and !pattern for positive and negative regexps.
Nested data structures are printed out in a legible fashion, unlike the print function.
The details of printout are governed by multiple Options.
$ 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): 42or, with the Option frame=2 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
b 237 $x > 30 b 237 ++$count237 < 11 b 33 /pattern/i
1. check for a breakpoint at this line 2. print the line if necessary (tracing) 3. do any actions associated with that line 4. prompt user if at a breakpoint or in single-step 5. evaluate lineFor example, this will print out $foo every time line 53 is passed:
a 53 print "DB FOUND $foo\n"
To disable this behaviour, set these values to 0. If dieLevel is 2,
then the messages which will be caught by surrounding eval are also
printed.
If frame & 4, arguments to functions are printed as well as the context and caller info. If frame & 8, overloaded stringify and tied FETCH are enabled on the printed arguments. If frame & 16, the return value from the subroutine is printed as well.
The length at which the argument list is truncated is governed by the
next option:
The following options affect what happens with V, X, and x
commands:
During startup options are initialized from $ENV{PERLDB_OPTS}. You can put additional initialization options TTY, noTTY, ReadLine, and NonStop there.
Example rc file:
&parse_options("NonStop=1 LineInfo=db.out AutoTrace");The script will run without human intervention, putting trace information into the file db.out. (If you interrupt it, you would better reset LineInfo to something ``interactive''!)
This module should implement a method new which returns an object
with two methods: IN and OUT, returning two filehandles to use
for debugging input and output correspondingly. Method new may
inspect an argument which is a value of $ENV{PERLDB_NOTTY} at
startup, or is "/tmp/perldbtty$$" otherwise.
Here's an example of using the $ENV{PERLDB_OPTS} variable:
$ PERLDB_OPTS="N f=2" perl -d myprogramwill run the script myprogram without human intervention, printing out the call tree with entry and exit points. Note that N f=2 is equivalent to NonStop=1 frame=2. Note also that at the moment when this documentation was written all the options to the debugger could be uniquely abbreviated by the first letter (with exception of Dump* options).
Other examples may include
$ PERLDB_OPTS="N f A L=listing" perl -d myprogram- runs script noninteractively, printing info on each entry into a subroutine and each executed line into the file listing. (If you interrupt it, you would better reset LineInfo to something ``interactive''!)
$ env "PERLDB_OPTS=R=0 TTY=/dev/ttyc" perl -d myprogrammay be useful for debugging a program which uses Term::ReadLine itself. Do not forget detach shell from the TTY in the window which corresponds to /dev/ttyc, say, by issuing a command like
$ sleep 1000000See the section on Debugger Internals below for more details.
Set an Option inhibit_exit to 0 if you want to be able to step
off the end the script. You may also need to set $finished to 0 at
some moment if you want to step through global destruction.
Currently the following setting are preserved: history, breakpoints,
actions, debugger Options, and the following command line
options: -w, -I, and -e.
|V main
= quit qor list current aliases.
DB<8>or even
DB<<17>>where that number is the command number, which you'd use to access with the builtin csh-like history mechanism, e.g., !17 would repeat command number 17. The number of angle brackets indicates the depth of the debugger. You could get more than one set of brackets, for example, if you'd already at a breakpoint and then printed out the result of a function call that itself also has a breakpoint, or you step into an expression via s/n/t expression command.
DB<1> for (1..4) { \ cont: print "ok\n"; \ cont: } ok ok ok okNote that this business of escaping a newline is specific to interactive commands typed into the debugger.
$ = main::infested called from file `Ambulation.pm' line 10 @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7 $ = main::pests('bactrian', 4) called from file `camel_flea' line 4The left-hand character up there tells whether the function was called in a scalar or list context (we bet you can tell which is which). What that says is that you were in the function main::infested when you ran the stack dump, and that it was called in a scalar context from line 10 of the file Ambulation.pm, but without any arguments at all, meaning it was called as &infested. The next stack frame shows that the function Ambulation::legs was called in a list context from the camel_flea file with four arguments. The last stack frame shows that main::pests was called in a scalar context, also from camel_flea, but from line 4.
Note that if you execute T command from inside an active use
statement, the backtrace will contain both require
frame and an eval) frame.
DB<<13>> l 101: @i{@i} = (); 102:b @isa{@i,$pack} = () 103 if(exists $i{$prevpack} || exists $isa{$pack}); 104 } 105 106 next 107==> if(exists $isa{$pack}); 108 109:a if ($extra-- > 0) { 110: %isa = ($pack,1);Note that the breakable lines are marked with :, lines with breakpoints are marked by b, with actions by a, and the next executed line is marked by ==>.
What follows is the start of the listing of
env "PERLDB_OPTS=f=n N" perl -d -Vfor different values of n:
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/nul: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/nul:0 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li in @=Config::myconfig() from /dev/nul: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), 'PATCHLEVEL') from lib/Config.pm:574 in $=Config::FETCH(ref(Config), '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/nul: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/nul: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/nul:0 out $=main::BEGIN() from /dev/nul:0 in @=Config::myconfig() from /dev/nul: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), 'PATCHLEVEL') from lib/Config.pm:574 out $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574 in $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/Config.pm:574
in $=main::BEGIN() from /dev/nul: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/nul: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/nul:0 out $=main::BEGIN() from /dev/nul:0 in @=Config::myconfig() from /dev/nul: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 the cases indentation of lines shows the call tree, if bit 2 of frame is set, then a line is printed on exit from a subroutine as well, if bit 4 is set, then the arguments are printed as well as 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 as well.
When a package is compiled, a line like this
Package lib/Carp.pm.is printed with proper indentation.
$DB::single = 1;If you set $DB::single to the value 2, it's equivalent to having just typed the n command, whereas a value of 1 means the s command. The $DB::trace variable should be set to 1 to simulate having typed the t command.
Another way to debug compile-time code is to start debugger, set a breakpoint on load of some module thusly
DB<7> b load f:/perllib/lib/Carp.pm Will stop on load of `f:/perllib/lib/Carp.pm'.and restart debugger by R command (if possible). One can use b compile subname for the same purpose.
You can do some customization by setting up a .perldb file which contains initialization code. For instance, you could make aliases like these (the last one is one people expect to be there):
$DB::alias{'len'} = 's/^len(.*)/p length($1)/'; $DB::alias{'stop'} = 's/^stop (at|in)/b/'; $DB::alias{'ps'} = 's/^ps\b/p scalar /'; $DB::alias{'quit'} = 's/^quit(\s*)/exit\$/';One changes options from .perldb file via calls like this one;
parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");(the code is executed in the package DB). Note that .perldb is processed before processing PERLDB_OPTS. If .perldb defines the subroutine afterinit, it is called after all the debugger initialization ends. .perldb may be contained in the current directory, or in the LOGDIR/HOME directory.
If you want to modify the debugger, copy perl5db.pl from the Perl library to another name and modify it as necessary. You'll also want to set your PERL5DB environment variable to say something like this:
BEGIN { require "myperl5db.pl" }As the last resort, one can use PERL5DB to customize debugger by directly setting internal variables or calling debugger functions.
A rudimentary command line completion is also available. Unfortunately, the names of lexical variables are not available for completion.
Perl is also delivered with a start file for making emacs act like a syntax-directed editor that understands (some of) Perl's syntax. Look in the emacs directory of the Perl source distribution.
(Historically, a similar setup for interacting with vi and the X11 window system had also been available, but at the time of this writing, no debugger support for vi currently exists.)
Meanwhile, you can fetch the Devel::Dprof module from CPAN. Assuming it's properly installed on your system, to profile your Perl program in the file mycode.pl, just type:
perl -d:DProf mycode.plWhen the script terminates the profiler will dump the profile information to a file called tmon.out. A tool like dprofpp (also supplied with the Devel::DProf package) can be used to interpret the information which is in that profile.
If perl is run with -d option, the following additional features
are enabled (cf. the section on $^P in the perlvar manpage):
Same for evaluated strings which contain subroutines, or which are
currently executed. The $filename for evaled strings looks like
(eval 34).
Note that if &DB::sub needs some external data to be setup for it to work, no subroutine call is possible until this is done. For the standard debugger $DB::deep (how many levels of recursion deep into the debugger you can go before a mandatory break) gives an example of such a dependency.
The minimal working debugger consists of one line
sub DB::DB {}which is quite handy as contents of PERL5DB environment variable:
env "PERL5DB=sub DB::DB {}" perl -d your-scriptAnother (a little bit more useful) minimal debugger can be created with the only line being
sub DB::DB {print ++$i; scalar <STDIN>}This debugger would print the sequential number of encountered statement, and would wait for your CR to continue.
The following debugger is quite functional:
{ package DB; sub DB {} sub sub {print ++$i, " $sub\n"; &$sub} }It prints the sequential number of subroutine call and the name of the called subroutine. Note that &DB::sub should be compiled into the package DB.
After the rc file is read, the debugger reads environment variable PERLDB_OPTS and parses it as a rest of O ... line in debugger prompt.
It also maintains magical internal variables, such as @DB::dbline, %DB::dbline, which are aliases for @{"::_<current_file"} %{"::_<current_file"}. Here current_file is the currently selected (with the debugger's f command, or by flow of execution) file.
Some functions are provided to simplify customization. See the section on Debugger Customization for description of DB::parse_options(string). The function DB::dump_trace(skip[, count]) skips the specified number of frames, and returns a list containing info about the caller frames (all if count is missing). Each entry is a hash with keys context ($ or @), sub (subroutine name, or info about eval), args (undef or a reference to an array), file, and line.
The function DB::print_trace(FH, skip[, count[, short]]) prints formatted info about caller frames. The last two functions may be convenient as arguments to <, << commands.
If you alter your @_ arguments in a subroutine (such as with shift or pop, the stack backtrace will not show the original values.
Say, 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 much 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 by another 20 bytes. A sloppy malloc() implementation will make these numbers yet more.
On the opposite end of the scale, a declaration like
sub foo;may take (on some versions of perl) up to 500 bytes of memory.
Off-the-cuff anecdotal estimates of a code bloat give a factor around 8. This means that the compiled form of reasonable (commented indented etc.) code will take approximately 8 times more than the disk space the code takes.
There are two Perl-specific ways to analyze the memory usage: $ENV{PERL_DEBUG_MSTATS} and -DL switch. First one is available only if perl is compiled with Perl's malloc(), the second one only if Perl compiled with -DDEBUGGING (as with giving -D optimise=-g option to Configure).
env 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 moment by usind Devel::Peek::mstats() (module Devel::Peek is available on CPAN).
Here is the explanation of different parts of the format:
The above line describes limits of buckets currently in use. Each bucket has two sizes: memory footprint, and the maximal size of user data which may be put into this bucket. Say, in the above example the smallest bucket is both sizes 4. The biggest bucket has usable size 8188, and the memory footprint 8192.
With debugging Perl some buckets may have negative usable size. This
means that these buckets cannot (and will not) be used. For greater
buckets the memory footprint may be one page greater than a power of
2. In such a case the corresponding power of two is printed instead
in the APPROX field above.
Say, with the above example the memory footprints are (with current algorith)
free: 8 16 32 64 128 256 512 1024 2048 4096 8192 4 12 24 48 80With non-DEBUGGING perl the buckets starting from 128-long ones have 4-byte overhead, thus 8192-long bucket may take up to 8188-byte-long allocations.
The amounts sbrk()ed by external libraries is not counted.
do 'lib/auto/POSIX/autosplit.ix';The file in question contains a header and 146 lines similar to
sub getcwd ;Note: the discussion below supposes 32-bit architecture. In the newer versions of perl the memory usage of the constructs discussed here is much improved, but the story discussed below is a real-life story. This story is very terse, and assumes more than cursory knowledge of Perl internals.
Here is the itemized list of Perl allocations performed during parsing of this file:
!!! "after" at test.pl line 3. Id subtot 4 8 12 16 20 24 28 32 36 40 48 56 64 72 80 80+ 0 02 13752 . . . . 294 . . . . . . . . . . 4 0 54 5545 . . 8 124 16 . . . 1 1 . . . . . 3 5 05 32 . . . . . . . 1 . . . . . . . . 6 02 7152 . . . . . . . . . . 149 . . . . . 7 02 3600 . . . . . 150 . . . . . . . . . . 7 03 64 . -1 . 1 . . 2 . . . . . . . . . 7 04 7056 . . . . . . . . . . . . . . . 7 7 17 38404 . . . . . . . 1 . . 442 149 . . 147 . 9 03 2078 17 249 32 . . . . 2 . . . . . . . .To see this list insert two warn('!...') statements around the call:
warn('!'); do 'lib/auto/POSIX/autosplit.ix'; warn('!!! "after"');and run it with -DL option. The first warn() will print memory allocation info before the parsing of the file, and will memorize the statistics at this point (we ignore what it prints). The second warn() will print increments w.r.t. this memorized statistics. This is the above printout.
Different Ids on the left correspond to different subsystems of perl interpreter, they are just first argument given to perl memory allocation API New(). To find what 9 03 means grep the perl source for 903. You will see that it is util.c, function savepvn(). This function is used to store a copy of existing chunk of memory. Using C debugger, one can see that it is called either directly from gv_init(), or via sv_magic(), and gv_init() is called from gv_fetchpv() - which is called from newSUB().
Note: to reach this place in debugger and skip all the calls to savepvn during the compilation of the main script, set a C breakpoint in Perl_warn(), continue this point is reached, then set breakpoint in Perl_savepvn(). Note that you may need to skip a handful of Perl_savepvn() which do not correspond to mass production of CVs (there are more 903 allocations than 146 similar lines of lib/auto/POSIX/autosplit.ix). Note also that Perl_ prefixes are added by macroization code in perl header files to avoid conflicts with external libraries.
Anyway, we see that 903 ids correspond to creation of globs, twice per glob - for glob name, and glob stringification magic.
Here are explanations for other Ids above:
It also creates a GV and a CV per subroutine (all called from
start_subparse()).
It also creates C arrays to keep data for the stash (this is one HV,
but it grows, thus there are 4 big allocations: the big chunks are not
freeed, but are kept as additional arenas for SV allocations).
Big allocations with this Id correspond to allocations of new
arenas to keep HE.
If warn() string starts with
If your perl is compiled with -DDEBUGGING, you may use the -Dr flag on the command line.
Otherwise, one can use re 'debug', which has effects both at compile time, and at run time (and is not lexically scoped).
compiling RE `[bc]d(ef*g)+h[ij]k$' size 43 first at 1 1: ANYOF(11) 11: EXACT <d>(13) 13: CURLYX {1,32767}(27) 15: OPEN1(17) 17: EXACT <e>(19) 19: STAR(22) 20: EXACT <f>(0) 22: EXACT <g>(24) 24: CLOSE1(26) 26: WHILEM(0) 27: NOTHING(28) 28: EXACT <h>(30) 30: ANYOF(40) 40: EXACT <k>(42) 42: EOL(43) 43: END(0) anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating) stclass `ANYOF' minlen 7The first line shows the pre-compiled form of the regexp, and the second shows the size of the compiled form (in arbitrary units, usually 4-byte words) and the label id of the first node which does a match.
The last line (split into two lines in the above) contains the optimizer info. In the example shown, the optimizer found that the match should contain a substring de at the offset 1, and substring gh at some offset between 3 and infinity. Moreover, when checking for these substrings (to abandon impossible matches quickly) it 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 the match cannot be shorter than 7 chars.
The fields of interest which may appear in the last line are
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 used to avoid entering (a slow) RE engine on strings which will definitely not match. If isall flag is set, a call to the RE engine may be avoided even when optimizer found an appropriate place for the match.
The rest of the output contains the list of nodes of the compiled form of the RE. Each line has format
id: TYPE OPTIONAL-INFO (next-id)
# TYPE arg-description [num-args] [longjump-len] DESCRIPTION
# Exit points END no End of program. SUCCEED no Return from a subroutine, basically.
# Anchors: BOL no Match "" at beginning of line. MBOL no Same, assuming multiline. SBOL no Same, assuming singleline. EOS no Match "" at end of string. EOL no Match "" at end of line. MEOL no Same, assuming multiline. SEOL no Same, assuming singleline. BOUND no Match "" at any word boundary BOUNDL no Match "" at any word boundary NBOUND no Match "" at any word non-boundary NBOUNDL no Match "" at any word non-boundary GPOS no Matches where last m//g left off.
# [Special] alternatives ANY no Match any one character (except newline). SANY no Match any one character. ANYOF sv Match character in (or not in) this class. ALNUM no Match any alphanumeric character ALNUML no Match any alphanumeric char in locale NALNUM no Match any non-alphanumeric character NALNUML no Match any non-alphanumeric char in locale SPACE no Match any whitespace character SPACEL no Match any whitespace char in locale NSPACE no Match any non-whitespace character NSPACEL no Match any non-whitespace char in locale DIGIT no Match any numeric character NDIGIT no Match any non-numeric character
# BRANCH The set of branches constituting a single choice are hooked # together with their "next" pointers, since precedence prevents # anything being concatenated to any individual branch. The # "next" pointer of the last BRANCH in a choice points to the # thing following the whole choice. This is also where the # final "next" pointer of each individual branch points; each # branch starts with the operand node of a BRANCH node. # BRANCH node Match this alternative, or the next...
# BACK Normal "next" pointers all implicitly point forward; BACK # exists to make loop structures possible. # not used BACK no Match "", "next" ptr points backward.
# Literals EXACT sv Match this string (preceded by length). EXACTF sv Match this string, folded (prec. by length). EXACTFL sv Match this string, folded in locale (w/len).
# Do nothing 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.
# STAR,PLUS '?', and complex '*' and '+', are implemented as circular # BRANCH structures using BACK. Simple cases (one character # per match) are implemented with STAR and PLUS for speed # and to minimize recursive plunges. # STAR node Match this (simple) thing 0 or more times. PLUS node Match this (simple) thing 1 or more times.
CURLY sv 2 Match this simple thing {n,m} times. CURLYN no 2 Match next-after-this simple thing # {n,m} times, set parenths. CURLYM no 2 Match 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.
# OPEN,CLOSE,GROUPP ...are numbered at compile time. OPEN num 1 Mark this point in input as start of #n. CLOSE num 1 Analogous to OPEN.
REF num 1 Match some already matched string REFF num 1 Match already matched string, folded REFFL num 1 Match already matched string, folded in loc.
# grouping assertions IFMATCH off 1 2 Succeeds if the following matches. UNLESSM off 1 2 Fails if the following matches. SUSPEND off 1 1 "Independent" sub-RE. IFTHEN off 1 1 Switch, should be preceeded by switcher . GROUPP num 1 Whether the group matched.
# Support for long RE LONGJMP off 1 1 Jump far away. BRANCHJ off 1 1 BRANCH with long offset.
# The heavy worker EVAL evl 1 Execute some Perl code.
# Modifiers MINMOD no Next operator is not greedy. LOGICAL no Next opcode should set the flag only.
# This is not used yet RENUM off 1 1 Group with independently numbered parens.
# 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.
If RE 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 RE which 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.