Credit card number validator?

validate.rb

# Do NOT assume this is production code.

class String
  def to_is
    self.chars.map(&:to_i)
  end
end

def luhn cc
  cc.
    reverse.
    to_is.
    each_with_index.
    map{|a,b|(a*(b%2+1)).to_s}.
    join.
    to_is.
    inject(&:+) % 10 == 0
end

def valid cc, len
  cc.length == len && luhn(cc)
end

cc = ARGV[0]
n = case cc
    when /^34/, /^37/
      puts 'AMEX'; 15
    when /^6011/
      puts 'Discover'; 16
    when /^5[1-5]/
      puts 'MasterCard'; 16
    when /^4/
      puts 'Visa'; [13, 16]
    else
      puts 'Unknown'; -1
    end
puts (Array(n).any?{|len| valid(cc, len)} ? 'Valid' : 'Invalid')