title: DataAnnotations validation adding `@` to RegularExpression
body: <p>How do I add to accept <code>@</code> along with the RegularExpression I have below?</p>
  
  <pre><code>[StringLength(250)]
  [RegularExpression(@"[A-Za-z0-9][A-Za-z0-9\-\.]*|^$", 
  ErrorMessage = "DomainName may only contain letters (a-z), digits (0-9), hypens (-) and dots (.), and must start with a letter or digit")]
  public string DomainName{ get; set; }
  </code></pre>