adv.with regret (used in polite formulas)

"I must regretfully decline your kind invitation"

Regretfully yours
REGRETFULLY, DICK
Regretfully Speaking
Bando-Ya regretfully bids syonara to Fresno's own culinary United Nations
Lighthouse Trails Regretfully No Longer Carrying Voice of the Martyrs Materials
Jesse Jackson Apologises for 'Regretfully Crude' Obama Remark; INTERNATIONAL
Woman Regretfully Recalls The Horror Of Her Abortion: “Then I Hear It, The Evil Vacuum. My Mind Screams No!”
Lithuanian-polish relations after 2004: Good old cooperation in regretfully bad new wrapping
Case study of a regretfully unusual pair: Exploration of possibilities and limitations of an international-domestic friendship at an...
Does Research in Performance Appraisal Influence the Practice of Performance Appraisal? Regretfully Not!