You cannot change the RewriteRule and also not change it 😉
If you want to support /shareboard/login -> controller=shareboard, action=users, id=login, then add a new RewriteRule to do that. That's all it takes. Because it's okay to have more than one of these as long as they match different URL patterns.
When you do that, please clean them up so less stuff is optional. Because you have a problem with being too flexible on what you're matching. Try going to some invalid URLs and see what happens.