SHANGHAI, Oct 22 (Reuters) - China’s commercial banks purchased a net $20.9 billion of foreign exchange in September, compared with a net purchase of $13.6 billion in August, the foreign exchange regulator said on Friday. For the January to September period, net foreign exchange purchases were $180 billion, the State Administration of Foreign Exchange said in a statement on its website. (Reporting by Beijing Monitoring Desk; Editing by Christian Schmollinger)