Represents a range of chars, from begin to end, included. So charRange!('a','z') matches
all English lowercase letters. If fails if the input is empty or does not begin with a character
between begin and end.
If begin == end, it will match one char (begin... or end).
Represents a range of chars, from begin to end, included. So charRange!('a','z') matches all English lowercase letters. If fails if the input is empty or does not begin with a character between begin and end.
If begin == end, it will match one char (begin... or end).
begin > end is non-legal.