Simple Javascript that adds "%" at end of entry if entry includes no "%"

I had this. I lost it. Now I can't replicate it. Help?

(FYI the code should exclude entries of "Share" and a "%" only. It should add a percent only to numbers 1-100)

I believe it was a custom format script looking like this:
if (event.value != "Share" && event.value != "%" && event.value == (/^[1-9][0-9]$|^[1-9]$|^100$/)) {event.target.value = event.value + "\%"}