How to restrict input box to take only numbers

<input type="text" name="card_num" size="4" maxlength="4" onkeyup="return isNumberKey(this);">


<script>
function isNumberKey(e)
{
 var numberRegex = /^[0-9]+$/;
 if(!numberRegex.test(e.value))
 {
   e.value = e.value.replace(/\D/g, '');
 }
}
</script>