army vs regular army

army and regular army both are nouns.

Word	Noun	Adjective	Verb	Adverb
army	Yes	No	No	No
regular army	Yes	No	No	No

As nouns, army and regular army are synonyms defined as:

army and regular army: a permanent organization of the military land forces of a nation or state

Other synonyms of army include ground forces.

army (noun)	regular army (noun)
a permanent organization of the military land forces of a nation or state	a permanent organization of the military land forces of a nation or state
a large number of people united for some specific purpose