Regrettably or regretfully?
Regrettably: adjective - undesirable, unwelcome, deserving censure; something deserving or worthy of regret.
Regretfully: adjective - Feeling or showing regret.
Something that is 'regrettable' is something undesirable or unfortunate.
If, on the other hand, someone is 'regretful', they are full of regret and wish they had not done or said whatever initiated the regret.
'Nancy's overindulgence in alcohol, which caused her to perform a spectacular striptease on the top table at her sister's wedding, was regrettable, especially since the groom posted a video of the event on Youtube.'
'Nancy was regretful that she'd allowed too much alcohol to influence her into performing a humiliating strip before all the guests at her sister's wedding, especially when the groom, who she was trying to impress, exposed her to the world by posting a video online.'
Pic: The sea front at Staithes, North Yorkshire.