gistfile1.builder

def mask(x)y=x.dup;y[6...-4]=y[6...-4].gsub(/./,'*');y end

mask_test.rb

[ ['Discover',            '6011111111111117', '601111******1117'],
  ['MasterCard',          '5555555555554444', '555555******4444'],
  ['Visa',                '4012888888881881', '401288******1881'],
  ['American Express',    '378282246310005',  '378282*****0005' ],
  ['Diners Club',         '30569309025904',   '305693****5904'  ],
].each do |card, number, masked|
  actual = mask(number)
  next if actual == masked
  puts "%20s %-20s => %-20s (expected %s)"%[card,number,actual,masked]
end