validate email input javascript onchange

 <script type="text/javascript">
 function ShowAlert() {
  var email = document.getElementById('txtEmailId');
  var filter = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;
    if (!filter.test(email.value)) {
        alert('Please provide a valid email address');
        email.focus;
        return false;
    }
    else {
        alert("Thanks for your intrest in us, Now you 
        will be able to receive monthly updates from us.");
        document.getElementById('txtEmailId').value = "";
    }
 }
 </script>    <input input type="text" name="txtEmailId" id="txtEmailId" /> 
   <input type="submit" class="button" value="Suscribe" name="Suscribe" 
            onclick="javascript:ShowAlert()" />