title: Regular expression that matches valid IPv6 addresses
body: <p>I'm having trouble writing a regular expression that matches valid IPv6 addresses, including those in their compressed form (with <code>::</code> or leading zeros omitted from each byte pair).  </p>
  
  <p>Can someone suggest a regular expression that would fulfill the requirement?</p>
  
  <p>I'm considering expanding each byte pair and matching the result with a simpler regex.</p>