Javascript dashes in phone number

First, clean your input by deleting all chars that are not numbers (ref.: Regex to replace everything except numbers and a decimal point)

Then, you put your dashes.

function addDashes(f)
{
    f_val = f.value.replace(/\D[^\.]/g, "");
    f.value = f_val.slice(0,3)+"-"+f_val.slice(3,6)+"-"+f_val.slice(6);
}

I have a strong tendency to treat phone numbers as a straight string of 10 digits with no formatting (so I can apply formatting to them on-the-fly, as needed and so searching and comparison is simpler), although that may change if I ever have to deal with international phone numbers. If all you're dealing with is US phone numbers, this will work nicely (formats it as it's typed):

function addDashes(f) {
    var r = /(\D+)/g,
        npa = '',
        nxx = '',
        last4 = '';
    f.value = f.value.replace(r, '');
    npa = f.value.substr(0, 3);
    nxx = f.value.substr(3, 3);
    last4 = f.value.substr(6, 4);
    f.value = npa + '-' + nxx + '-' + last4;
}​

Here's a fiddle: http://jsfiddle.net/EYuk5/