VS 2012 Creating list based on format?

User gives format x#, suggested output:
That is an easy task with nested for-loops. But what about dynamic-length formats? I want the user to be able to input a format and I can make a full list based on their input. (inputting either x for [a-z], X for [A-Z], # for [0-9]).

I know how to do this with a fixed-length 'format' but I'm rather lost when it comes to dynamic-length 'format's.

Thanks.