31313...32323 (108-digits)

This number is a prime.

                                   31313131 3131313131 3131313131 3131313131
3131313123 2323232323 2323232323 2323232323 2323232323 2323232323 2323232323

Twenty-three 31's followed by thirty-one 23's is prime. [De Geest]