Question

Thanks guys. I take your advice and modify the script as following. Now it seems working.

<script src="jquery-1.4.2.min.js" type="text/javascript"></script>   
<script>
$(function(){
var pat=/^[0-9]{5}$/;
if ( !pat.test(   $('#zip').val()   ) )
{$('#zip').after('<p>zip is invalid</p>');}
})
</script>

zip (US only) <input type="text" name='zip' id='zip' maxlength="5">

Answer 1

.val needs to be .val()
and use .after() or .before() and not .append()

var pattern = /^[0-9]{5}$/;
if (!pattern.test($('#zip').val())) {
  $('#zip').after('<p>zip is invalid</p>');
}