Xiànzài cái xiǎngqǐ lái, chī hòuhuǐyào yě méiyòng le.
Only thinking of it now - there's no point in regretting it anymore.
Tips
culture
Often used in the negative: 没有后悔药可吃 / 没卖后悔药 ('no regret-pill exists / no one sells regret-pills'), meaning what's done can't be undone. The imaginary 'regret pill' is the punchline.
memory
Picture a pharmacy where 后悔药 (regret pills) are eternally out of stock. The phrase only works because that medicine doesn't exist.
