AngularJS Discovery Log - Form Validations

It's been a long time since I last worked on something besides work, so this past weekend I decided to get back to my expense tracking app after realizing I needed it now more than ever.  While working on it, I ran into a small dilemma: form validations.  I understand you can create directives, or even use the built-in filters if they fulfill your needs; but what if you needed more than that?  I asked Google sensei about validation libraries that exist for AngularJS, and there were a few such as Angular-Validator.  Although Angular-Validator looked like it would help with many of the validations I was looking for, there were some validations that I decided could only be done with directives.  I needed to be able to run some business logic that would validate the passed parameter to make sure it met requirements before allowing it to be accepted.  I am still currently trying to figure this out so no code samples at the moment, but hopefully in the future.