This is valid `r'[A-Za-z0-9]'`, so is this `r'[0-A]'` - I would like to know what characters class is being matched here. 

What is the underlying character table for Python's (2.7) `re` module?
Do `[A-Z]`, `[a-z]`, `[0-9]` reference separate tables or is a single, *whole*, indexed table such as [ASCII](http://www.asciitable.com)? What defines the character table(s) to be used?

I am asking to understand the underlying mechanism.