decline vs regret

decline and regret both are nouns.

Word	Noun	Adjective	Verb	Adverb
decline	Yes	No	Yes	No
regret	Yes	No	Yes	No

As verbs, regret is a hyponym of decline; that is, regret is a word with a more specific, narrower meaning than decline:

Other hyponyms of decline include disdain, freeze off, pooh-pooh, reject, scorn, spurn, turn down, contract out, repudiate, disobey.

decline (noun)	regret (noun)
a downward slope or bend	sadness associated with some wrong done or some disappointment
a gradual decrease; as of stored charge or current
change toward something smaller or lower
a condition inferior to an earlier condition; a gradual falling off from a better state; decline

decline (verb)	regret (verb)
grow smaller	decline formally or politely
grow worse	express with regret
show unwillingness towards	feel remorse for; feel sorry for; be contrite about
inflect for number, gender, case, etc.	feel sad about the loss or absence of
go down
fall in value
not accept as true