#!/usr/bin/perl -w
use strict;

my $string = '0-0-0-23-34-2345-345-21-0-0-0-256-78-0-0-0-0-0-0-0-56-45-3-34-0-2-3-4-5-6-0-0-0-0-0-0';

while ($string=~/((-\d{1,5}){4,20}(-0){4,20})/g) {
    print pos($string),"\n";
}

##</code><code>##

#!/usr/bin/perl -w
use strict;

my $string = '0-0-0-23-34-2345-345-21-0-0-0-256-78-0-0-0-0-0-0-0-56-45-3-34-0-2-3-4-5-6-0-0-0-0-0-0';

for my $match ($string =~/(?<!\d)(?:[^0]\d*-)+(?:0-){3,}0/g) {
    print "$match\n";
}

##</code><code>##

 
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (?<!                     look behind to see if there is not:
----------------------------------------------------------------------
    \d                       digits (0-9)
----------------------------------------------------------------------
  )                        end of look-behind
----------------------------------------------------------------------
  (?:                      group, but do not capture (1 or more times
                           (matching the most amount possible)):
----------------------------------------------------------------------
    [1-9]+                   any character of: '1' to '9' (1 or more
                             times (matching the most amount
                             possible))
----------------------------------------------------------------------
    -                        '-'
----------------------------------------------------------------------
  )+                       end of grouping
----------------------------------------------------------------------
  (?:                      group, but do not capture (at least 3
                           times (matching the most amount
                           possible)):
----------------------------------------------------------------------
    0-                       '0-'
----------------------------------------------------------------------
  ){3,}                    end of grouping
----------------------------------------------------------------------
  0                        '0'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------