>perl -wMstrict -le
"my @suits = qw(D S C H);
 my $suits = join '', @suits;
 my $flush = qr{ ([$suits]) (?: [^$suits]+ \1){4} }xms;
 print $_, $_ =~ $flush ? ' flush' : ' nope'
   for '2H 3D 4C 5S 5H', '1H 2H 3H 4H 5H';
"
2H 3D 4C 5S 5H nope
1H 2H 3H 4H 5H flush