;;; Based on RFC5321 (Simple Mail Transfer Protocol)
;;; https://tools.ietf.org/rfc/rfc5321.txt
;;; + Added support for RFC6532 via RFC6531 (SMTP Extension for Internationalized Email)
;;;   https://tools.ietf.org/rfc/rfc6532.txt
;;;   https://tools.ietf.org/rfc/rfc6531.txt
;;; + Added stricter IP address grammar
;;;   https://tools.ietf.org/html/rfc3986#appendix-A
;;; + No longer accept General-address-literal

;; Address specification

; Mailbox specification

mailbox         =   local-part "@" domain

; Domain specification

domain          =   domain-name / address-literal

domain-name     =   sub-domain *("." sub-domain)

sub-domain      =   let-dig [ldh-str] / U-Label

U-Label         =   1*UTF8-non-ascii

; Original RFC5321 definition of
; ```
; Let-dig        = ALPHA / DIGIT
; Ldh-str        = *( ALPHA / DIGIT / "-" ) Let-dig
; address-literal =   "[" ( IPv4-address-literal / IPv6-address-literal / General-address-literal ) "]"
; ```
; changed for better compatability with ABNFC parser generator.
; Following definitions are supposed to be equal to them

let-dig         =   ALPHA / DIGIT

hyphen-let-dig  =   *("-") let-dig

ldh-str         =   1*(hyphen-let-dig / let-dig)

address-literal =   "[" IPv4-address-literal "]" / "[" IPv6-address-literal "]"

; Local part specification

local-part      =   dot-string / quoted-string

dot-string      =   atom *("."  atom)

quoted-string   =   DQUOTE *qcontentSMTP DQUOTE

qcontentSMTP    =   qtextSMTP / quoted-pairSMTP

quoted-pairSMTP =   %d92 %d32-126   ; i.e., backslash followed by any ASCII
                                    ; graphic (including itself) or SPace

qtextSMTP       =   %d32-33 /       ; i.e., within a quoted string, any
                    %d35-91 /       ; ASCII graphic or space is permitted
                    %d93-126 /      ; without blackslash-quoting except
                    UTF8-non-ascii  ; double-quote and the backslash itself.

; Atom

atext           =   ALPHA / DIGIT / ; Printable US-ASCII
                    "!" / "#" /     ; characters not including
                    "$" / "%" /     ; specials. Used for atoms.
                    "&" / "'" /
                    "*" / "+" /
                    "-" / "/" /
                    "=" / "?" /
                    "^" / "_" /
                    "`" / "{" /
                    "|" / "}" /
                    "~" / UTF8-non-ascii

atom            =   1*atext

;; Internationalized Email Headers

UTF8-non-ascii  =   UTF8-2 / UTF8-3 / UTF8-4

; UTF-8 Byte Sequences (https://tools.ietf.org/rfc/rfc3629.txt)

UTF8-2          =   %xC2-DF UTF8-tail

UTF8-3          =   %xE0 %xA0-BF UTF8-tail / %xE1-EC 2( UTF8-tail ) /
                    %xED %x80-9F UTF8-tail / %xEE-EF 2( UTF8-tail )

UTF8-4          =   %xF0 %x90-BF 2( UTF8-tail ) / %xF1-F3 3( UTF8-tail ) /
                    %xF4 %x80-8F 2( UTF8-tail )

UTF8-tail       =   %x80-BF

;; SMTP Address Literals

; Original RFC5321 definitions of
; ```
;   IPv4-address-literal     =   Snum 3("." Snum)
;   IPv6-addr                =   IPv6-full /
;                                IPv6-comp /
;                                IPv6v4-full /
;                                IPv6v4-comp
;   IPv6-hex                 =   1*4HEXDIG
;   IPv6-full                =   IPv6-hex 7(":" IPv6-hex)
;   IPv6-comp                =   [IPv6-hex *5(":" IPv6-hex)] "::"
;                                [IPv6-hex *5(":" IPv6-hex)]
;   IPv6v4-full              =   IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal
;   IPv6v4-comp              =   [IPv6-hex *3(":" IPv6-hex)] "::"
;                                [IPv6-hex *3(":" IPv6-hex) ":"]
;                                IPv4-address-literal
; ```
; Discarded in favor of those defined in [RFC3986 Appendix A]
; (https://tools.ietf.org/html/rfc3986#appendix-A).
; They are much stricter and friendlier for ABNFC parser generator.

IPv4-address-literal     =   dec-octet 3("." dec-octet)

dec-octet                =   "25" %x30-35          ; 250-255
                           / "2" %x30-34 DIGIT     ; 200-249
                           / "1" 2DIGIT            ; 100-199
                           / %x31-39 DIGIT         ; 10-99
                           / DIGIT                 ; 0-9

IPv6-address-literal     =   "IPv6:" IPv6-addr

IPv6-addr                =                               6( H16 ":" ) LS32
                           /                        "::" 5( H16 ":" ) LS32
                           / [ H16                ] "::" 4( H16 ":" ) LS32
                           / [ H16 0*1( ":" H16 ) ] "::" 3( H16 ":" ) LS32
                           / [ H16 0*2( ":" H16 ) ] "::" 2( H16 ":" ) LS32
                           / [ H16 0*3( ":" H16 ) ] "::"    H16 ":"   LS32
                           / [ H16 0*4( ":" H16 ) ] "::"              LS32
                           / [ H16 0*5( ":" H16 ) ] "::"              H16
                           / [ H16 0*6( ":" H16 ) ] "::"

H16                      =   1*4HEXDIG

LS32                     =   ( H16 ":" H16 ) / IPv4-address-literal

;; RFC4234 CORE (For abnfc binary compatability)

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

DIGIT           =   %x30-39             ; 0-9

DQUOTE          =   %x22                ; " (Double Quote)

HEXDIG          =   DIGIT / "A" / "B" / "C" / "D" / "E" / "F" ; 0-9 A-F