title: Ruby: how to check if an UTF-8 string contains only letters and numbers?
body: <p>I have an UTF-8 string, which might be in any language.</p>
  
  <p>How do I check, if it does not contain any non-alphanumeric characters?</p>
  
  <p>I could not find such method in UnicodeUtils Ruby gem.</p>
  
  <p>Examples:</p>
  
  <ol>
  <li>ėččę91 - valid</li>
  <li>$120D - invalid</li>
  </ol>