There is a RFC 4234 for ABNF (Augmented BNF). No eBNF though.

ALPHA          =  %x41-5A / %x61-7A   ; A-Z / a-z

BIT            =  "0" / "1"

CHAR           =  %x01-7F
                       ; any 7-bit US-ASCII character,
                       ;  excluding NUL