遗憾 (yí hàn) is a noun meaning 'regret' (a feeling about something past) and an adjective meaning 'regrettable'. It is more formal and deeper than 可惜 (kě xī, 'it's a pity'), which is lighter and often about missed opportunities. Unlike 后悔 (hòu huǐ, 'to regret' an action), 遗憾 is not a verb and does not take an object.