title: Regular expression to match a character that can only appear once in a string?
body: <p>I'm looking for a regular expression that will match if the string contains the character <code>*</code>, but <strong>only once</strong>. It should match <code>a*aa</code>, <code>aa*aaaaa</code>, <code>a*aaaa</code>, but it should not match <code>a**a</code>, <code>a****</code>, <code>****</code>.</p>
  <p>Any advice?</p>