悔やむ
くやむ · kuyamu · JLPT N2 · verb-godan
to regret
悔 (regret) + やむ (be pained, as in 病む) = to regret or mourn
The second part, やむ, shares a root with 病む (to fall ill), so the word carries the sense of being pained by something that cannot be undone. The kanji 悔 also appears in 後悔 (regret).