Here's a little Perl snippet which will place commas in any number string right where they belong:

sub commify
{
    my $text = reverse $_[0];
    $text =~ s/(\d\d\d)(?=\d)(?!\d*\.)/$1,/g;
    return scalar reverse $text;
}

So for example, by calling commify("13245766") the value "13,245,766" will be returned.

All the little commas right exactly at the correct positions using a simple regular expression. Pretty amazing language that Perl is. Does it make sense?