Select specific character positions from string

I'm looking for a way to select pseudo random characters from a string.

For example, I have a 64 character string. I would like to pick positions 0, 1, 4, 5, 8, 9.

Or a harder one would be with the same string, I would pick positions 0, 1, 2, 4, 6, 8, 10, 11, 12 and so on.

Is there a quick way to do this?