There's no use regretting what has already happened; it's too late to undo it.
試験に落ちてから後悔先に立たず、もっと早く対策をとればよかった。
It’s no use regretting failing the exam now; I should have prepared earlier.
飲酒運転で取り返しのつかない事故を起こしたら、後悔先に立たずだ。
If you cause an irreversible accident by drunk driving, regret won’t help.
ミスを発見したらすぐに報告しよう。後悔先に立たずだから。
Report mistakes immediately. Regretting later won’t help.