Form app.alert help

Question

So I have this custom formating script:
// Custom format script for text field
if (!event.value) {
    event.value = event.target.userName;
    event.target.display = display.noPrint;
    event.target.textColor = color.red;
} else {
    event.target.display = display.visible;
    event.target.textColor = color.black;
    event.value = util.printx("99-9999999", event.value);
if (event.value.length !=10) {
event.value = "";
app.alert("Tax ID # does not have the required 9 numbers!", 0, 0)
So if you don't input the right format and hit tab then the little app alert pops up and after you click okay it moves you to the next field (since you hit tab).
My question is how to make it if the app.alert is executed then to make the previous tab request null and keep you in the field that caused the app alert to pop up?
I hope that makes sense if not let me know and I'll try to clarify.

Answer 1

The alert doesn't do any checks, it just shows an alert. If you mean that you want to make sure the user entered exactly 9 numbers, you would need to change the last part of the code to something like this:
if (event.value.length != 9) {
event.value = "";
app.alert("Tax ID # does not have the required 9 numbers!", 0, 0)
event.target.setFocus();
} else {
event.value = util.printx("99-9999999", event.value);