noun, noun which may take the genitive case particle 'no'
no regrets; without regrets
Used to express that one has no regrets about a past action or decision. Often appears in set phrases like 悔いなしの人生.
彼は悔いなしの人生を送った。
He lived a life with no regrets.
この選択に悔いなし。
I have no regrets about this choice.
後悔 is a more common noun/verb for 'regret', while 悔いなし is a set phrase meaning 'no regrets'.
Compound of 悔い (regret) + なし (without). The exact historical derivation is uncertain, but the phrase is conventionally associated with the meaning 'no regrets'.