happen upon vs regain

happen upon and regain both are verbs.

Word	Noun	Adjective	Verb	Adverb
happen upon	No	No	Yes	No
regain	No	No	Yes	No

As verbs, regain is a hypernym of happen upon; that is, regain is a word with a broader meaning than happen upon:

happen upon: find unexpectedly
regain: come upon after searching; find the location of something that was missed or lost

Other hypernyms of happen upon include find.

happen upon (verb)	regain (verb)
find unexpectedly	get or find back; recover the use of
	come upon after searching; find the location of something that was missed or lost