>perl -wMstrict -le
"my $a = 'AAAAXRXAAAAAXKXAAAAAXRPXAAA';
 my $cut = qr{ [KR] (?!P) }xms;
 my $digest = qr{ .*? $cut | (?<= $cut) .* \z }xms;
 my @digested = $a =~ m{ $digest }xmsg;
 print qq{'$_'} for @digested;
 $a = 'AAAXXXPPPAAAXXXAAAPPPAAA';
 @digested = $a =~ m{ $digest }xmsg;
 print '@digested empty' unless @digested;
"
'AAAAXR'
'XAAAAAXK'
'XAAAAAXRPXAAA'
@digested empty