Let's try this again. List ate my post. Maybe it's the S/MIME sig?

----

OK, this is the first Quiz contribution I have made (publicly). I  
think it is a good balance between terse and readable.

JEG2, the luhn_valid? check is what I mean by Functional Programming  
in Ruby, or as you say, the Power of Iterators. :-)

--be

####
#!/usr/bin/env ruby -wKU

# Some utility functions first
require 'enumerator'
module Enumerable
   # Maps n-at-a-time (n = arity of given block) and collects the  
results
   def mapn(&b)
     r = []
     each_slice(b.arity) {|*args| r << b.call(*args) }
     r
   end

   def sum; inject(0){|s, i| s + i} end
end

class CreditCardNumber < String
   TYPES = {"3[47]\\d{13}"       => "Amex",
            "6011\\d{12}"        => "Discover",
            "5[1-5]\\d{14}"      => "Mastercard",
            "4(\\d{12}|\\d{15})" => "Visa"}

   # Returns the type of the given card, or nil if the card does not  
match a pattern
   def card_type
     (t = TYPES.detect{|re, t| /^#{re}$/ === self}) && t.last
   end

   # Returns true iff Luhn check passes for this number
   def luhn_valid?
     # a trick: double_and_sum[8] == sum_digits(8*2) == sum_digits 
(16) == 1 + 6 == 7
     double_and_sum = [0, 2, 4, 6, 8, 1, 3, 5, 7, 9]
     split(//).reverse.mapn{|a,b| a.to_i + double_and_sum 
[b.to_i]}.sum % 10 == 0
   end

   # Returns true iff card matches a known type and Luhn check passes.
   def valid?
     luhn_valid? && !card_type.nil?
   end
end

if (arg = ARGV.join.gsub(/[^0-9]/, '')) and !arg.empty?
   number = CreditCardNumber.new(arg)

   if number.valid?
     puts "Valid #{number.card_type}"
   else
     puts "Invalid card"
   end
end