Changing the rules in the middle of the game?

Next time give exact desired output when you pose a problem.

Sorry, not meant to mislead.

#!/usr/bin/perl -l

# http://perlmonks.org/?node_id=1168288

use strict;
use warnings;

my @ra = qw(0001 0002 0003 011 012 013 015 16 17 18 20);

$_ = join ',', @ra;

s/\b(\d+)(?{$1})\K(?:,(\d+)\b(??{++$^R!=$2||length$1!=length$2}))+/-$2
+/g;

print;
[download]

WOW, that is perfect. Big Thanks. Can you give a brief explanation of the syntax if you don't mind? Again thanks.

#!/usr/bin/perl -l

# http://perlmonks.org/?node_id=1168288

use strict;
use warnings;

my @ra = qw(0001 0002 0003 011 012 013 015 16 17 18 20);

$_ = join ',', @ra;

s/\b               # find a number boundary
  (\d+)            # and a number $1
  (?{$1})          # save the number in $^R
  \K               # then leave the above in the string
  (?:              # non-capture grouping
    ,              # comma - also forces $1 to be a complete number
    (\d+)          # a complete number (because of , and \b)
    \b             # forces comlete number
    (??{           # extended pattern
      ++$^R != $2  # false if correct number
      ||
      length$1 != length$2 # false if same length
      })           # false (and therefor matches) if both false, other
+wise fails
                   # if true returns 1 which can't match because of th
+e \b
  )+               # repeat at least once
  /-$2/gx;         # replace second or more with last matching number

print;
[download]

I got this error.

Complex regular subexpression recursion limit (32766) exceeded

s/\b(\d+)(?{$1})\K(?:,(\d+)\b(??{++$^R!=$2||length$1!=length$2}))+/-$2
+/g;
[download]

Regex problems.