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

APADP
KGSTIDRPDAA
RTLTVH
KCEQTDT
RGV
KEGT
RNEDPQAECKPVSDVEFTIT
KLNVD

##</code><code>##

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

APADPK
GSTIDRPDAAR
TLTVHK
CEQTDTR
GVK
EGTR
NEDPQAECKPVSDVEFTITK
LNVD