4. ABNF
The following ABNF [Crocker] gives the formal syntax for Y10K years.
The initial characters definitions are given in their lexical collation (ASCII) order.
exclamation := '!'
star := '*'
slash := '/'
digit := 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
letter := A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z
caret := '^'