title: Regex with Letters, Numbers, @ and spaces
body: <p>I would like to know how to write a regex in c# that allows only
  numbers, letters, spaces and the <code>@</code> symbol.</p>
  
  <p>Valid inputs are:</p>
  
  <ul>
  <li><code>Abc</code></li>
  <li><code>Abc def</code></li>
  <li><code>@Abc</code></li>
  <li><code>Abc@</code></li>
  </ul>
  
  <p>Example I tried so far: <code>@"[^\w-\s-@]"</code></p>