Many thanks for your letter of inbitation,but much to my regret, I will not be able to honour it because I have been suffering from an old disease since this summer.

我十分感谢您的邀请信，但十分抱歉的是，我不能出行，因为从今年夏天开始，我的老毛病犯了。

youdao

I. Write a love Letter expressing your feelings of anger, sadness, fear, regret, and love.

写一封表达你的气愤、伤心、害怕、后悔和爱的感觉的情书。

youdao

In reply to your letter of August 8, we regret to say that our clients find your price too high.

现回复你方8月8日来信，很遗憾我方客户认为你方价格太高。

youdao

In that letter, Chinese Vice Premier Qien Qichen said U.S. statements of regret were not enough and held fast to China's demand for an outright U.S. apology.

CNN: briefing