Problem Statement

Idiomatically determine all the lowercase and uppercase letters   (of the Latin [English] alphabet)   being used currently by a computer programming language. The method should find the letters regardless of the hardware architecture that is being used (ASCII, EBCDIC, or other).

Display the set of all: that can be used (allowed) by the computer program, where   letter   is a member of the Latin (English) alphabet:     a ──► z     and     A ──► Z.

You may want to mention what hardware architecture is being used, and if applicable, the operating system.

Let's start with the solution:

enum chartypes
   LOWER = -1, UPPER = 1, NOTLETTER = 0
end enum

function letter_case( ch as string ) as byte
    'exploits the fact that ucase and lcase consider non-letters to be
    'both upper and lower case
    if ucase(ch)=lcase(ch) then return NOTLETTER
    if ch = ucase(ch) then return UPPER
    return LOWER
end function

dim as uinteger i
for i = 0 to 255
    if letter_case(chr(i)) = LOWER then print chr(i);
next i
print
for i = 0 to 255
    if letter_case(chr(i)) = UPPER then print chr(i);
next i
print