title: Regular Expression for Credit card information should not allow characters
body: <p>I am using a regular expression <code>^4[0-9]{15}$</code> to check credit card information starting with <code>4</code> for VISA and that are 16 digits long.</p>
  
  <p>This expression accepts credit card numbers like <code>4141414141414141aa</code> which is wrong, as it accepts alpha-characters in the end of a valid credit card number.</p>
  
  <p>How do I fix this?</p>