title: Replace all non digits with an empty character in a string
body: <pre><code>public static String removeNonDigits(final String str) {
     if (str == null || str.length() == 0) {
         return "";
     }
     return str.replaceAll("/[^0-9]/g", "");
  }
  </code></pre>
  
  <p>This should only get the Digits and return but not doing it as expected! Any suggestions?</p>