37 C.F.R. § 42.101 : Patents, Trademarks, and Copyrights — UNITED STATES PATENT AND TRADEMARK OFFICE, DEPARTMENT OF COMMERCE — GENERAL PATENTS — TRIAL PRACTICE BEFORE THE PATENT TRIAL AND APPEAL BOARD — INTER PARTES REVIEW —GENERAL — Who may petition for inter partes review

A person who is not the owner of a patent may file with the Office a petition to institute an inter partes 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;
(b) The petition requesting the proceeding is filed more than one year after the date on which the petitioner, the petitioner's real party-in-interest, or a privy of the petitioner is served with a complaint alleging infringement of the patent; or
(c) 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.

37 C.F.R. §42.101