Whether c is a digit in base 16 (0 .. 9, A .. F, a .. f).
1 assert( isHexDigit('0')); 2 assert( isHexDigit('A')); 3 assert( isHexDigit('f')); // lowercase hex digits are accepted 4 assert(!isHexDigit('g')); 5 assert(!isHexDigit('G')); 6 assert(!isHexDigit('#'));