var isNN = (navigator.appName.indexOf("Netscape") != -1);
    
    function autoTab(input,len, e) {
        var keyCode = (isNN) ? e.which : e.keyCode; 
        var filter = (isNN) ? [0,8,9] : [0,8,9,16,17,18,37,38,39,40,46];

        if(input.value.length >= len && !containsElement(filter,keyCode)) {
            input.value = input.value.slice(0, len);
            input.form[(getIndex(input)+1) % input.form.length].focus();
        }

    function containsElement(arr, ele) {
        var found = false, index = 0;

        while(!found && index < arr.length)
            if(arr[index] == ele)
                found = true;
            else
                index++;
        return found;
    }
        
    function getIndex(input) {
        var index = -1, i = 0, found = false;

        while (i < input.form.length && index == -1)
            if (input.form[i] == input)index = i;
            else i++;
            return index;
        }
    return true;
    }