title: How to only match a single instance of a character?
body: <p>Not quite sure how to go about this, but basically what I want to do is match a character, say <code>a</code> for example. In this case all of the following would not contain matches (i.e. I don't want to match them):</p>
  
  <ul>
  <li><code>aa</code></li>
  <li><code>aaa</code></li>
  <li><code>fooaaxyz</code></li>
  </ul>
  
  <p>Whereas the following would:</p>
  
  <ul>
  <li><code>a</code> (obviously)</li>
  <li><code>fooaxyz</code> (this would only match the letter a part)</li>
  </ul>
  
  <p>My knowledge of RegEx is not great, so I am not even sure if this is possible. Basically what I want to do is match any single <code>a</code> that has any other non <code>a</code> character around it (except for the start and end of the string).</p>