[ adv ] with regret (used in polite formulas) <adv.all> I must regretfully decline your kind invitation
Regretful \Re*gret"ful\ (-f?l), a. Full of regret; indulging in regrets; repining. -- {Re*gret"ful*ly}, adv.
I have therefore regretfully concluded that it is in the best interests of the Government for me to resign my office without further ado.
His greatest success was "Asa Branca," or "White Wing," about a peasant regretfully fleeing the drought-stricken backlands but pledging to return when the rains came.