Regular- imperfect, impolite, impatient, improper, inactive, inconvenient, incorrect, inconsiderate, preview, prejudice, prevent, prefix, prehistoric, unable, uncertain, uncomfortable, unaware, ungrateful
Challenge- punctual, ancient, miniature, stupendous, inquisitive, wasteful, zealous, obnoxious, pathetic, obsolete, ridiculous, genuine, congenial, optimistic, reliable, abundant, meager, sparse