后悔莫及 is formed by combining the two-character phrase 后悔, meaning "regret", which itself consists of 后 (after) and 悔 (regret), with the two-character phrase 莫及, meaning "cannot reach", comprising 莫 (not) and 及 (reach). The idiom literally sequences these components to express the idea that regret occurs after an event and is now beyond remedy.