title: Regular expression to match repeated strings
body: <p>I am trying to make a regex that will </p>
  
  <ul>
  <li>match if the string exclusively is constructed with strings from a set of strings.</li>
  <li>not match if there is any other string in there.</li>
  </ul>
  
  <p>examples for a set of strings that is ['xyz', 'a', 'b']:</p>
  
  <ul>
  <li>'xyzab' == true</li>
  <li>'xyzxyzabbb' == true</li>
  <li>'aaabb' == true </li>
  <li>'' == true</li>
  <li>'d' == false</li>
  <li>'aabbbbd' == false</li>
  <li>'zxy' == false</li>
  </ul>
  
  <p>I am URL matching :/</p>