title: Function to determine if a string contains characters only from another
body: <p>I'm working on a validation function for html input tags. I've the string of the value of the particular input and the string containing the allowed characters.</p>
  
  <pre><code>var allowed = 'abcdefghijklmnopqrstuvwxyz";
  var value = element.value;
  </code></pre>
  
  <p>I'd like to write a function to determine if the <code>value</code> contains character only from the <code>allowed</code> string. I'm looking for a straight-forward and simple solution.
  Any ideas?</p>