In reply to janudeep3:
A much simpler solution is to use the streaming operator ( IEEE 1800-2012 section 11.4.14 Streaming operators (pack/unpack) )
module tb;
string s1 = "string";
string s2;
initial begin
s2 = {<<8{s1}};
$display("reversed string=%s", s2);
end
endmodule