regardless vs disregardless

regardless is an adjective but disregardless is not an adjective.

Word	Noun	Adjective	Verb	Adverb
regardless	No	Yes	No	Yes
disregardless	No	No	No	Yes

As adverbs, regardless and disregardless are synonyms defined as:

regardless and disregardless: in spite of everything; without regard to drawbacks

Other synonyms of regardless include disregarding, irrespective, no matter.

regardless (adjective)	disregardless (adjective)
(usually followed by `of') without due thought or consideration

regardless (adverb)	disregardless (adverb)
in spite of everything; without regard to drawbacks	in spite of everything; without regard to drawbacks