"regret deeply" is correct and usable in written English.
You can use it when expressing deep sorrow for a mistake or wrongdoing. Example: I regret deeply causing so much pain to my loved ones.
It's the one thing I regret deeply".
"It is something that I regret deeply that anyone had to go through.
"Of course I regret deeply and profoundly the loss of life," he said.
And I regret deeply that there has been these kinds of matters occurring.
I regret deeply he did not live to accept the honor of election to the Hall of Fame.
BAN: I regret deeply that during my time I was not able to see the end of this.
On its website, MVRDV issued the following statement: "MVRDV regrets deeply any connotations The Cloud project evokes regarding 9/11.
At the inquiry, Mr Blair said he "regretted deeply and profoundly the loss of life" during and after the 2003 conflict.
Tony Blair has said he "regretted deeply and profoundly the loss of life" in Iraq since the 2003 invasion.
"On behalf of the whole organisation, we regret this deeply and apologise unreservedly.
