Shell Programming and Scripting

#!/usr/bin/perl

use strict;
use warnings;

print ipv6_expand($ARGV[0]);

sub ipv6_expand
{
    local($_) = shift;
    chomp($_);
    s/^:/0:/;
    s/:$/:0/;
    s/(^|:)([^:]{1,3})(?=:|$)/$1.substr("0000$2", -4)/ge; # I don't understand this line
    my $temp = tr/:/:/;
    s/::/":".("0000:" x (8-$temp))/e;
    return $_;
}

(^|:)       => the beginning of the string or a ":" character (assign to $1) 
([^:]{1,3}) => followed by 1, 2 or 3 occurrences of a character other than ":" (assign to $2)
(?=:|$)     => followed by a ":" character or end of string (but do not assign to $3 or anything)

 $1.substr("0000$2", -4) => create a string by left-padding four zeros to $2 and then extracting 4 characters from the right hand side. Append this string to $1

(?=:|$)