She told me regrettably that she couldn't attend the party.

regrettably 是副词，由动词 regret（感到遗憾）加上形容词后缀 -able（可...的）变成形容词 regrettable（令人遗憾的），再加上副词后缀 -ly 构成。意思是"遗憾地、可惜地"，表示说话人带着遗憾的心情。