遺恨[ikon] grudge
怨恨[enkon] enmity
悔恨[kaikon] regret
逆恨み[sakaurami] being resented in return
旧恨[kyuukon] old grudge

恨み[urami] resentment
恨みっこ無し[uramikkonashi] with no hard feelings
恨みっこのないように[uramikkononaiyouni] making it even
恨み言[uramigoto] grudge
恨む[uramu] to resent

遺憾[ikan] regrettable
憾み[urami] resentment
憾む[uramu] to resent
遺憾に堪えない[ikannitaenai] really regrettable

遺憾なく[ikannaku] amply
遺憾千万[ikansenban] highly regrettable
憾むらくは[uramurakuha] I regret that