It is with regret that I have to send this communication.

对必须发出此通讯一事，我深感遗憾。

youdao

It is with great regret that I accept your resignation.

接受你的辞呈，我感到非常遗憾。

《牛津词典》

A final point: the word whippersnapper is a favourite of mine. It is deliciously evocative. My advice that I give with lingering regret: avoid.

最后一点：“自以为是的年轻人”这个字眼我很喜欢。它很容易引起共鸣。但我不得不遗憾地建议：避免用它。

youdao

"It has become increasingly difficult for Mr Dreyfuss to fulfil the rigours of the role, and it is with regret that both parties have had to make this decision, " the statement said.

BBC: NEWS | Entertainment | Dreyfuss pulls out of Producers

It is with deep regret that I have tendered my resignation following the decisions of the standards committee this week.

BBC: Wendy Alexander

It is with enormous regret that I have decided to resign as controller of BBC Radio 2, BBC 6 Music and of Popular Music.

BBC: Lesley Douglas resignation letter