title: Single repetition for some of the symbols in a set of characters
body: <p>I need a regular expression that matches a string containing letters and the symbols: <code>@</code>, <code>.</code> or space. But all the symbols must appear only once in the whole string.</p>
  
  <ul>
  <li><p><code>^[@][.][a-z]+$</code> - This matches for <code>example@.asdf</code> but need something to match one <code>@</code> and one dot in the string.</p></li>
  <li><p><code>^[a-z]+[@][a-z]+[.][a-z]+$</code> - This is the best result for now. </p></li>
  </ul>
  
  <p>I was just wondering if I can use something like that - <code>^[a-z[@]{1}]$</code>.</p>