Elementary probability theory.

Each position has 1/58 probability to be the desired symbol.
So if you fix let say 3 positions the probability to find a matching pattern would be   (1/58)*(1/58)*(1/58)
It is independent of the position you fix in the address. 1ace....xxxx is as difficult as 1axcxex.......

The possibility vanishes very quick as a power of n(positions you fix)