regrettably vs unfortunately

regrettably

adv
  • In a manner inspiring or deserving regret. 

unfortunately

adv
  • Used (as a parenthetical word) to express disappointment, compassion, sorrow, regret or grief. 

  • Happening through bad luck, or because of some unfortunate event. 

How often have the words regrettably and unfortunately occurred in a corpus of books? (source: Google Ngram Viewer )