Javascript Validation

Question

HiI have gone through few forms and tutorials before writing this post.In one of my forms, I need to validate a field such that:a. It should not be emptyb. It should be numbers onlyc. It should be exactly 8 digitsI believe a and b can be done using editor but not c. So I wrote a JS to perform all the three. But not getting how and where to place it.1. Placed onSubmit=" return checkForm(this)" in Form tag attachment field2. Placed below code in Javascript box under form code tab
function checkForm(form)
{
alert ("jai");
var elem = document.getElementById('text_13');
alert (elem);
var numericExpression = /^[0-9]+$/;
	if(elem.value.length == 0){
		alert("Customer Number must be filled out");
		elem.focus();
		return false;
	}

else if(!elem.value.match(numericExpression)){
alert("Customer Number must be numbers only");
		elem.focus();
		return false;
}
else if (elem.value.length !=8)
{
alert("Customer Number must be eight digits");
		elem.focus();
		return false;

}

return true;
}
I have just placed alerts to know whether function is being called or not. But I am not getting any alerts. The form validation is working even if select No in validation tab. The form is online at  http://www.canadabreadfoodservice.ca/lang/promotion-entry-form/madeincanada.html. Any help please