title: How to check form input in html form control
body: <p>I have an html file which contains a form.  There is lots of text typed input control need customer to input information.  How do I check whether the customer's input is correct?
  For example, I want to check if the username only contains digits, letters and '_'.</p>
  
  <p>Please help me.  </p>
  
  <p>I'd appreciate if somebody can provide me a demo.</p>
  
  <p>Thanks in advance! </p>