Character constants are written as single characters enclosed in single quotes. You can use the relational operators (<, <=, >, >=, ==) to sort characters by ASCII order. You can cast a character to a real to get its ASCII value. You can cast a real ASCII value to a character.
(int) 'A' == 65 (char) 65 == 'A' is_digit x = '0' <= x && x <= '9'