Rueful is an adjective meaning feeling or showing regret or sorrow. To express the adverbial sense of regretfully or sorrowfully, you can use phrases like:
* With rue
* Regretfully
* Sorrowfully
* Sadly
* Penitently
For example, you could say:
* "He looked at her with rue."
* "She spoke regretfully about her past mistakes."
* "He sighed sorrowfully."