use strict;
use warnings;
use IPC::Run3;
use Regexp::Compare qw( is_less_or_equal );

my @re = (
  qr/asdf/,
  qr/asd/,
  qr/as/,
  qr/a/,
  qr/./,
  qr/.*/,
);

my $stdin  = '';
my $stdout = '';
my $stderr = '';
for my $i ( 0 .. $#re ) {
  for my $j ( 0 .. $#re ) {
    $stdin .= "$i $j\n" if is_less_or_equal( $re[$i], $re[$j] );
  }
}

# tsort (GNU coreutils) 8.4
run3 [ 'tsort' ], \$stdin, \$stdout, \$stderr;
print $stderr;

my @idx = split /\s+/, $stdout;
print "$_\n" for @re[@idx];

##</code><code>##

my @re = (
  qw/./,
  qw/./,
  qw/.+/,
);

##</code><code>##

in $stderr:
tsort: -: input contains a loop:
tsort: 0
tsort: 2
tsort: 1
tsort: -: input contains a loop:
tsort: 0
tsort: 2
tsort: -: input contains a loop:
tsort: 1
tsort: 2

##</code><code>##

hypothesis:
                  $re1 <= $re2
is_less_or_equal( $re1,   $re2 )
      specific -> $re1    $re2 <- generic

if exists $str that
$str =~ /$re1/ and $str =~ /$re2/ -> no consequence
$str =~ /$re1/ and $str !~ /$re2/ -> falsifies hypothesis
$str !~ /$re1/ and $str =~ /$re2/ -> corroborates hypothesis
$str !~ /$re1/ and $str !~ /$re2/ -> no consequence