A person who is not the owner of a patent may file with the Office a petition to institute a post-grant review of the patent unless:

(a) Before the date on which the petition for review is filed, the petitioner or real party-in-interest filed a civil action challenging the validity of a claim of the patent; or

(b) The petitioner, the petitioner's real party-in-interest, or a privy of the petitioner is estopped from challenging the claims on the grounds identified in the petition.