(comparative more regretfully, superlative most regretfully)
In careful usage, regretfully means with regret (in a manner expressed with regret, expressing remorse), while regrettably means deserving regret (sadly, unfortunately), and in the body of sentences this distinction is observed: John regretfully asked for forgiveness, not *John regrettably asked for forgiveness, and The weather was regrettably terrible, not *The weather was regretfully terrible. These terms are occasionally conflated, a practice noted and decried since Fowler 1926 (in the forms regretful and regrettable).
However, in use as a sentence adverb, these are sometimes used interchangeably to mean “unfortunately”, a practice noted since the 1960s in the United States, as in: Regrettably, it is raining or Regretfully, it is raining, the latter being proscribed by some. This is similar to and possibly influenced by the use of hopefully, which predates this usage of regretfully, and is both far more popular than regretfully and has no precise equivalent, *hopeably not existing.
regretful + -ly