88888...99999 (39-digits)

This number is a prime.

                                  888888885 5555777777 7444466666 6999999999

The largest prime concatenated from the six largest rep-digit numbers, each of them formed by using d times the digit d, i.e., 4444, 55555, 666666, 7777777, 88888888, 999999999. [Loungrides]