turn back vs regress

turn back is not a noun while regress is a noun.

Word	Noun	Adjective	Verb	Adverb
turn back	No	No	Yes	No
regress	Yes	No	Yes	No

As verbs, turn back and regress are synonyms defined as:

Other synonyms of turn back include retrovert, return, revert.

As verbs, regress is a hyponym of turn back; that is, regress is a word with a more specific, narrower meaning than turn back:

Other hyponyms of turn back include fall back, lapse, recidivate, relapse, retrogress, resile, go back, recover, recuperate.

turn back (noun)	regress (noun)
	returning to a former state
	the reasoning involved when you assume the conclusion is true and reason backward to the evidence

turn back (verb)	regress (verb)
go back to a previous state	go back to bad behavior
turn inside out or upside down	get worse or fall back to a previous condition
hold back, as of a danger or an enemy; check the expansion or influence of	go back to a previous state
retrace one's course	go back to a statistical means
force to go away; used both with concrete and metaphoric meanings