#!/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;
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