Actually, we both were buggy. Using your code and my code (with the appropriate bugfix), we have: 
#!/usr/bin/perl
 
use strict;
use warnings;
 
my $x = "12345678";
my $y = "123456789";
 
sub dragonchild
{
   my ($n, $s) = @_;
 
   $n = reverse $n;
   # Added negative lookahead
   $n =~ s/(.{4})(?!$)/${1}${s}/g;
   scalar reverse $n;
}
 
sub aristotle
{
   my ($n, $s) = @_;
   scalar reverse join $s, (reverse $n) =~ /(....)/g;
}
 
foreach my $v ($x, $y)
{
   print dragonchild($v, '-'), $/;
   print aristotle($v, '-'), $/;
}

------

1234-5678
1234-5678
1-2345-6789
2345-6789

[download]

Your code only works for card numbers that are a multiple of 4. Most department store cards are 11 digits and some, like AmEx, are moving to 17 or 19 digits. (I used to work for MC ... we had a similar function that I had to maintain in the app I worked on. It was written horribly with substr and the like.)

------
We are the carpenters and bricklayers of the Information Age.

The idea is a little like C++ templates, except not quite so brain-meltingly complicated. -- TheDamian, Exegesis 6

Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.

In reply to Re3: Simple formatting question for credit card number by dragonchild
in thread Simple formatting question for credit card number by Anonymous Monk

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