Show something if another field validates

I have an input box which needs to have something entered. The validation is using a regex and it works fine. The input box is NOT in a form, it doesn’t need to be as I only want to check the value of it, nothing is ever submitted. But, there is a link which I only want to show if the value in the input box is validated. It shows the validation error correctly but I don’t know how to hide the link if the validation fails (or show the link if it succeeds).

Any ideas?