title: how to negate a RegEx match using Data annotations in asp.net mvc?
body: <p>I have this expression</p>
  
  <pre><code>[RegularExpression(@"(https?://)?([\da-z.-]+)\.([a-z.]{2,6})([/\w .-]*)*/?", ErrorMessage = "Message cannot contain urls")]
  </code></pre>
  
  <p>It detects the urls as expected, but I need it to do the opposite.
  The validation is only warning if I do NOT enter a url in the field. I need it to give me a warning if it detects the url saying that urls and links are not allowed.</p>