regret

1. Origins, structure, and basic meaning

• Structure: 悔 has 9 strokes, the on-reading かい, and the kun-reading くやしい among its readings. 悔しい (くやしい) describes frustration or a sense of unfairness.
• Basic usage: 悔しい concerns frustration at a defeat or misunderstanding; 後悔 (こうかい) concerns regret about a past action.

2. Common pitfalls and similar expressions

• Frustration, unfairness, and regret
  • 一点差で負けたのが本当に悔しい means that losing by one point is deeply frustrating. 誤解されたまま謝るのは悔しかった describes feeling it was unfair to apologize while still misunderstood.
  • 謝らなかったことを今も後悔している means still regretting not having apologized. 後悔のないよう means so as to have no regrets.