$s = join'', map{ chr( 65+rand(26) ) x rand( 100 ) } 1 .. 1000;;

cmpthese( -1,{
    a=>q[ 
        1 while $s =~ m[((?=(.))\2+)]g;
    ],
    b=>q[
        1 while $s =~ m[((?=(.))\2+)]sg;
    ] ,
    c=>q[
        1 while $s =~ m/((.)\2*)/sg;
    ], 
    d=>q[
        1 while $s =~ m{
                        (
                            (?=(.))
                            (?:
                              (\2\2\2\2\2\2\2\2) \3*
                            | (\2\2\2\2) \4*
                            | (\2\2) \5*
                            | \2+
                            )+
                        )
                      }sgx;
    ], 
});

   Rate    b    a    c    d
b 193/s   --  -0%  -3% -35%
a 194/s   0%   --  -2% -35%
c 198/s   3%   2%   -- -33%
d 296/s  54%  53%  50%   --