title: How to restrict a textbox only to enter a hexadecimal value
body: <p>I have a textbox in my HTML form, and I would like to restrict a user to enter a hexadecimal value in it between 0 and f. How can I do it?</p>
  
  <p>My HTML textbox code:</p>
  
  <pre><code>&lt;div class="input-resp-hpux-vbus"&gt;&lt;span&gt;&lt;input  class="textbox" id="vbus-id" name="vbus-id" type="text" size="5" maxlength="3"  value="&lt;?php if(isset($_POST['vbus-id'])) { echo htmlentities ($_POST['vbus-id']); }?&gt;" /&gt;&lt;/span&gt;&lt;/div&gt;
  </code></pre>