title: How can I check with a regex that a string contains only certain allowed characters?
body: <p>I need a special regular expression and have no experience in them whatsoever, so I am turning to you guys on this one.</p>
  
  <p>I need to validate a classifieds title field so it doesn't have any special characters in it, almost.</p>
  
  <p>Only letters and numbers should be allowed, and also the three Swedish letters å, ä, ö (upper- or lowercase).</p>
  
  <p>Besides the above, these should also be allowed:</p>
  
  <ul>
  <li>The "&amp;" sign.</li>
  <li>Parentheses "()"</li>
  <li>Mathematical signs "-", "+", "%", "/", "*"</li>
  <li>Dollar and Euro signs</li>
  <li>One accent signed letter: "é". // Only this one is required</li>
  <li>Double quote and single quote signs.</li>
  <li>The comma "," and point "." signs</li>
  </ul>