11115...51111 (121-digits)

This number is a prime.

                    1 1115115511 1111515111 1115555115 5551151115 1115115155
5515115151 5151151555 5151151115 1115115555 1155551111 1151511111 1551151111

                      1
                     1 1
                    1 5 1
                   1 5 5 1
          1 1 1 1 1 5 1 5 1 1 1 1 1
           1 5 5 5 5 1 1 5 5 5 5 1
            1 5 1 1 1 5 1 1 1 5 1
             1 5 1 5 5 5 5 1 5 1
              1 5 1 5 1 5 1 5 1
             1 5 1 5 5 5 5 1 5 1
            1 5 1 1 1 5 1 1 1 5 1
           1 5 5 5 5 1 1 5 5 5 5 1
          1 1 1 1 1 5 1 5 1 1 1 1 1
                   1 5 5 1
                    1 5 1
                     1 1
                      1

A star-congruent prime with alternating bands of two different digits [Hartley]