79999...99999 (77-digits)

This number is a prime.

                                                                     7999999
9999999999 9999999999 9999999999 9999999999 9999999999 9999999999 9999999999

The largest near-repdigit prime p less than a googol of the form p = 8*10^n - 1, (case n=76). [Bajpai]