Regretfully rhymes with:
artfully, delightfully, fitfully, frightfully, gratefully, respectfully, rightfully, tactfully, tastefully, thoughtfully, wistfully
Regretfully sounds like:
recordable, regretful, regrettable, regrettably
artfully, delightfully, fitfully, frightfully, gratefully, respectfully, rightfully, tactfully, tastefully, thoughtfully, wistfully
recordable, regretful, regrettable, regrettably