title: Regex Expression on 16-digit number
body: <p>I am stuck on this regex problem.</p>
  
  <p>A 16-digit credit card number, with the first digit being a 5 and the second digit being a 1, 2, 3, 4, or 5 (the rest of the digits can be anything). </p>
  
  <p>so far I have <code>^4[1,5]\d{14}</code> and I know I'm missing a lot of things but I dont know what I'm missing..</p>
  
  <p>please help and thanks!</p>