Format Phone Numbers onKeyup

HTML

<input id="phone" type="text" name="phone" maxlength="14" onKeyup="formatPhone(this);" />

JavaScript

function formatPhone(obj) {
    var numbers = obj.value.replace(/\D/g, ''),
        char = {0:'(',3:') ',6:'-'};
    obj.value = '';
    for (var i = 0; i < numbers.length; i++) {
        obj.value += (char[i]||'') + numbers[i];
    }
}