input:     RYOURPOSTBROKEN
expecting: R YOURPOSTBR OK EN

split (?-xism:(?<=[KR])(?!P)): R YOURPOSTBR OK EN
(?s-xim:(.*?[KR])(?!P)): R YOURPOSTBR OK
(?s-xim:[^KR]+.(?!P)): YOU POSTBR OK EN
(?s-xim:[^KR]*.(?!P)): R YOU POSTBR OK EN