Adverb
with regret (used in polite formulas)
Source: WordNetWhen one door closes, another opens; but we often look so long and so regretfully upon the closed door that we do not see the one which has opened for us. Alexander Graham Bell
I must regretfully decline your kind invitation Source: Internet
He changed his mind after finding out he would be returned to the same jail in Washington, D.C. He regretfully said: "My conscience tells me I should have gone (to jail), but I was shaking all over. Source: Internet
A touch regretfully, to my mind, the contest is not known as the Dirt Bowl"). Source: Internet
The Doctor regretfully informs her that there is no difference in forethought. Source: Internet
While we were tempted by the $15 bottomless mimosa deal, the fact that we (regretfully) needed to drive after dissuaded us. Source: Internet