By Reuters Staff 3 Min Read (Adds response from regulator in paragraph 7 and 8; clarifies curbs as verbal guidance in paragraph 3 and 10) SHANGHAI/BEIJING, March 12 (Reuters) - Regulators have tightened the screws on foreign exchange transactions in one of China’s main export hubs by restricting conversions of exporters’ dollar earnings, people familiar with the matter said, as authorities seek to rein in a surging yuan. China has long held a tight grip on its currency due to concerns that excessive volatility could affect capital flows and harm the economy. Unlike other large economies, Beijing does not allow fully unrestricted cross-border money transfers, and foreign currency cannot be freely converted into yuan.