how to limit field entries to 1 or 3?

Hi -

I need a very simple java script that will limit a field's entries to 1 or 3. I believe this is a custom validation script.

I don't know why acrobat requires us to use javascript for something so basic, but apparently they do, so I'd greatly appreciate any help on how to do this.

If it's just a matter of putting in a standard shell and then using a GREP expression, can someone tell me how to set it up so I can customize it in the future?

Learning javascript has been on my to do list for a while, but if I need to use it for something like this I think I'll have to push it to the top of the list.

I've seen other scripts on the forums to do other sorts of field validation but can't tell how much of it I need to include for this.

Thanks in advance for the help.