Ano toki, motto benkyou shite okeba yokatta to koukai shite iru.

I regret that I should have studied harder back then.

Ichidokiri no jinsei dakara, koukai no nai you ni ikitai.

Since we only live once, I want to live my life with no regrets.

Kare wa jibun no sentaku o fukaku koukai shite iru you da.

He seems to deeply regret his choice.

Junbi o okotaru to, ato de koukai suru koto ni narimasu yo.

If you neglect your preparation, you will end up regretting it later.

Donna kekka ni narou tomo, watashi wa kesshite koukai shimasen.

No matter what the outcome may be, I will never regret it.

Sunda koto o ikura koukai shite mo, kako wa kaerarenai.

No matter how much you regret what is done, you cannot change the past.