print for split '(?=[KR][^P])', 
'APADPKGSTIDRPDAARTLTVHKCEQTDTRGVKEGTRNEDPQAECKPVSDVEFTITKLNVD';;

APADP
KGSTIDRPDAA
RTLTVH
KCEQTDT
RGV
KEGT
RNEDPQAECKPVSDVEFTIT
KLNVD