hex_digit 
<hex_digit> ::=
0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
| A | B | C | D | E | F
| a | b | c | d | e | f
hex_digit <hex_digit> ::=
0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
| A | B | C | D | E | F
| a | b | c | d | e | f