Antonyms of irregular :
regular, timed, official, well-ordered, rule-governed, first-string, full-time, stock, standard, prescribed, perfect, uniform, even, normal, continual, lawful, conventional, weak, standing, common, commonplace, everyday, familiar, ordinary, routine, run-of-the-mill, run-of-the-mine, unexceptional, unremarkable, workaday, customary, usual, wonted, archetypal, average, characteristic, representative, natural, typical, changeless, equable, stable, stationary, unchanging, unvarying, unwavering, methodical, orderly, systematic, unrelenting, unremitting, constant, continuous, habitual, periodic, repeated, steady, exact, aligned, true, horizontal, tabular, plumb, straight, vertical, flush, flat, level, plane, smooth, correct, decorous, proper, constrained, inhibited, restrained, rigid, stiff, stuffy, uptight, ceremonial, ceremonious, formal, orthodox.
Examples of usage:
First, Geoffrey's private conference with Sir Patrick on the subject of Irregular Marriages in Scotland. "Man and Wife", Wilkie Collins.
He passed but one building here, and that seemed large and hall- like, though irregular. "Shirley", Charlotte BrontÃ«.
It consists of two scenes in irregular verses, dealing with Jupiter's love for the mortal Semele' and Juno's jealousy. "The Life and Works of Friedrich Schiller", Calvin Thomas.