regardless has 5 distinct synonyms spread across 2 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
(usually followed by `of') without due thought or consideration
“careless of the consequences”
in spite of everything; without regard to drawbacks
“he carried on regardless of the difficulties”
Synonyms, antonyms and the broader and narrower relations for regardless come from Princeton University’s WordNet. Full sources and licences.