RecruitingPhase 2Drug trialZanubrutinib With Pemetrexed to Treat Relapsed/Refractory Primary and Secondary Central Nervous System (CNS) LymphomasThis study is being conducted to evaluate the safety and efficacy of the combination of pemetrexed and zanubrutinib (called induction therapy) followed by zanubrutinib treatment alone (also called maintenance therapy) in people who have relapsed or refractory (RR) primary central nervous system lymphoma (PCNSL) or isolated central nervous system relapse of B cell lymphoma (SCNSL). Assessments include how well people respond to this treatment, whether their disease gets better or worse, and their survival. Safety of this treatment and its side effects also will be assessed.