てしまう / ちゃう

1. 1Expresses that an action is completely finished or done with.Ex. 1
2. 2Expresses that something happened unintentionally or regrettably, often with a feeling of 'oops' or 'I ended up doing it.'Ex. 2

The full form てしまう works in both speech and writing. ちゃう / じゃう is casual and common in everyday conversation and informal writing such as texts or social media posts, though not in formal settings. Context usually decides whether completion or regret is meant, and words like 全部 and もう can support either reading. The regret nuance is implied, whereas ばよかった states outright what the speaker wishes they had done.

• きるN3「きる」attaches to the ます-stem and is close to 「てしまう」 only in the sense of 'completely finishing,' with stronger emphasis on persisting to the very end and leaving nothing behind; it does not express regret or unintended results, and can also indicate a state reaching its extreme, as in 「疲れきる」.
• 終わるN4「終わる」attaches to the verb's ます-stem and only objectively indicates that a continuous action has run to its end, with no emotional coloring or sense of regret; it is close to 「てしまう」 only in the 'finished doing' sense and cannot attach to punctual verbs.
• たばかりN4「たばかり」attaches to the た-form and emphasizes that not much time has passed since completing something or entering a new state—a subjective 'just did...'; 「てしまう」attaches to the て-form and emphasizes finishing or something being beyond recall, with no reference to elapsed time. The two cannot be interchanged.
• 出すN4「出す」attaches to the verb's ます-stem and focuses on the very moment an action or phenomenon begins, often with a sense of suddenness, as in 「泣き出す」; 「てしまう」attaches to the て-form and focuses on the completion of an action or an irreversible result. Their meanings differ, so they cannot be interchanged.
• ばよかったN3「ばよかった」attaches to the ば-form and expresses regret by imagining how much better it would have been if the past had been different; when 「てしまう」expresses regret, it directly states something that has already happened and cannot be undone. Compare 「食べてしまった」and 「食べなければよかった」—the two cannot be interchanged.

Unlike きる (doing something thoroughly or to the very end) and 終わる (neutrally finishing an ongoing action), てしまう adds a sense of finality or regret; unlike 出す (suddenly starting an action), it concerns completion rather than onset. たばかり stresses that an action happened only a short time ago rather than that it is done or regrettable, and ばよかった explicitly states what one should have done, while てしまう only implies regret.