title: Regex Odd/Even Amount
body: <p>I have a regex question that I do not know how to do. 
  It has to match all strings consisting any number of a's at the beginning and then either a single 0 if the number of a's was even or a single 1 if the number of a's was odd.</p>
  
  <p>How can you keep track of even/odd?</p>
  
  <p>Sample</p>
  
  <ul>
  <li>aaa1</li>
  <li>aaaa0</li>
  </ul>