How to partial complete text input

I know the title doesn't capture what I'm trying to ask but here is what I'm trying to do:

// desired format is: 123-123-1234

When the user enters 123 (the first three characters) how can I append the hyphen - Then when the user enters 123 (the second three characters) how can I append the hyphen - again And then allow the user only to ente开发者_Go百科r the last four numbers?

I have a function that validates the US Phone number format I want:

/* US Phone # */
'us_phonenumber':function($fld) {
    var v = $fld.val();

    if(v.length == 0) {
        return true;
    }
    return /^\d{3}-\d{3}-\d{4}$/.test(v);
},

Just curious if I could add some sort of partial auto complete method while still validating

HTML:

<input type="text" />
<br /><a href="#" id="check">Click me!</a>

JS:

$("#check").click(function(){
    var pNum = $("input").val();
    var vNum = pNum.slice(0, 3)+"-"+pNum.slice(3, 6)+"-"+pNum.slice(6, 10);
    $("input").val(vNum);
    return false;
});

Fiddle: http://jsfiddle.net/ueyhE/

Type in your number, then press "Click me!".

EDIT:

Using change():

$("input").click(function(){
    var pNum = $(this).val();
    var vNum = pNum.slice(0, 3)+"-"+pNum.slice(3, 6)+"-"+pNum.slice(6, 10);
    $(this).val(vNum);
});

i don't know about doing during input, but after the input is complete might be less confusing for the user, or less margin for error.

var string = '1234567890';
var formattedString = '';
var index;
for (var i = 0; i < string.length; i++) {
    index = string.charAt(i);   
    (i == 3 || i == 6) ? formattedString = formattedString + '-' + index : formattedString = formattedString + index;
}
alert(formattedString);