title: regular expression matching a 3 or 4 digit cvv of a credit card
body: <p>I really don't know regexp yet, but sooner or later I will read and learn. 
  But for now I need a regular expression matching a 3 or 4 digit cvv of a credit card so I can validate the input in javascript and later in PHP.
  cvv are supposed to be 3 or 4 digit as far as I know.</p>
  
  <p>I tried this in my javascript code and I am not sure if it is correct. </p>
  
  <pre><code>if (/[0-9]{3}+/.test(value))
  return false;
  </code></pre>