combine 3 fields with javascript

I'm new to javascript and was hoping someone could help me with this issue. Couldn't find existing code.

I have a simple form that requires the end user to submit their phone number. I would like to have 3 fields, area code, 3 digits and then final 4 digits in their own field. I would then like to combine the numbers into one variable before after submitting and before passing the action.

Why go thru the headache of three boxes & then adding them together into one field when you can do 1 input box, check it with a regular expression & then send in the one field? Sounds a lot easier huh?

Make a text box lets say called phoneNum & then check it like this with javascript:

function phoneTest() {
with document.forms[0] {

var pattern2 = /\d{3}\-\d{3}\-\d{4}/

if (!pattern2.test(phoneNum.value) {
alert("The phone number you entered is not in the proper format");
phoneNum.focus();
return false();
}
}
}

Of course dont be a jerk when setting up the input box, show them the format they are supposed to enter next to the input box.