How to use Regex in a nonmandatory custom field?

Hello UNA Team,

I have created a website url field (text field) which is not mandatory. I would like to add a regular expression to remove the http//, https// and www. from the url. The problem is Regex can only be used in UNA if the field is made mandatory.

How can I use a regex in a nonmandatory custom field? I would like that regex to be applied only if something is entered in that field.

Thanks