Form Validation - Only US Zip Code Allowed

I have created a javascript for form validation. But I want to add some more functions in it. Like it should allow only US zipcodes. Phone Number will be in Number format only. I referred many other sites as well. But its getting difficult to add those new codes in my program.

Here is my Javascript Code:
function validate(){
if(document.Enquiry.FirstName.value==""){
alert("Please enter Name");
document.Enquiry.FirstName.focus();
return false;
}if(document.Enquiry.Address.value==""){
alert("Please enter Your Postal Address");
document.Enquiry.Address.focus();
return false; .....