Regretfully (adverb)
In a regretful manner, with regret.
Regretfully (adverb)
Unfortunately, in a manner inspiring or deserving regret; used only as a sentence adverb (to introduce and modify an entire sentence).
Regrettably (adverb)
In a manner inspiring or deserving regret.
Wiktionary
Regrettably (adverb)
unfortunately (used to express apology for or sadness at something)
“regrettably, last night’s audience was a meagre one”
Oxford Dictionary