How can force the user/OS to input an Ascii string

This is an extended question of this one: Is std::string suppose to have only Ascii characters

I want to build a simple console application that take an input from the user as set of characters. Those characters include 0->9 digits and a->z letters.

I am dealing with input supposing that it is an Ascii. For example, I am using something like : static_cast<unsigned int>(my_char - '0') to get the number as unsigned int.

How can I make this code cross-platform? How can tell that I want the input to be Ascii always? Or I have missed a lot of concepts and static_cast<unsigned int>(my_char - '0') is just a bad way?

P.S. In Ascii (at least) digits have sequenced order. However, in others encoding, I do not know if they have. (I am pretty sure that they are but there is no guarantee, right?)