         ; Extracted from RFC5234

         rulelist       =  1*( rule / (*c-wsp c-nl) )

         rule           =  rulename defined-as elements c-nl
                                ; continues if next line starts
                                ;  with white space

         rulename       =  ALPHA *(ALPHA / DIGIT / "-")

         defined-as     =  *c-wsp ("=" / "=/") *c-wsp
                                ; basic rules definition and
                                ;  incremental alternatives

         elements       =  alternation *c-wsp

         c-wsp          =  WSP / (c-nl WSP)

         c-nl           =  comment / CRLF
                                ; comment or newline

         comment        =  ";" *(WSP / VCHAR) CRLF

         alternation    =  concatenation
                           *(*c-wsp "/" *c-wsp concatenation)

         concatenation  =  repetition *(1*c-wsp repetition)

         repetition     =  [repeat] element

         repeat         =  1*DIGIT / (*DIGIT "*" *DIGIT)

         element        =  rulename / group / option /
                           char-val / num-val / prose-val

         group          =  "(" *c-wsp alternation *c-wsp ")"

         option         =  "[" *c-wsp alternation *c-wsp "]"

         ;  char-val       =  DQUOTE *(%x20-21 / %x23-7E) DQUOTE
         ;                         ; quoted string of SP and VCHAR
         ;                         ;  without DQUOTE

         ; From RFC 7405:
         char-val       =  case-insensitive-string /
                           case-sensitive-string

         case-insensitive-string =
                           [ "%i" ] quoted-string

         case-sensitive-string =
                           "%s" quoted-string

         quoted-string  =  DQUOTE *(%x20-21 / %x23-7E) DQUOTE
                                ; quoted string of SP and VCHAR
                                ;  without DQUOTE

         num-val        =  "%" (bin-val / dec-val / hex-val)

         bin-val        =  "b" 1*BIT
                           [ 1*("." 1*BIT) / ("-" 1*BIT) ]
                                ; series of concatenated bit values
                                ;  or single ONEOF range

         dec-val        =  "d" 1*DIGIT
                           [ 1*("." 1*DIGIT) / ("-" 1*DIGIT) ]

         hex-val        =  "x" 1*HEXDIG
                           [ 1*("." 1*HEXDIG) / ("-" 1*HEXDIG) ]

         prose-val      =  "<" *(%x20-3D / %x3F-7E) ">"
                                ; bracketed string of SP and VCHAR
                                ;  without angles
                                ; prose description, to be used as
                                ;  last resort

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

         BIT            =  "0" / "1"

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

         CR             =  %x0D
                                ; carriage return

         CRLF           =  CR LF
                                ; Internet standard newline

         CTL            =  %x00-1F / %x7F
                                ; controls

         DIGIT          =  %x30-39
                                ; 0-9

         DQUOTE         =  %x22
                                ; " (Double Quote)

         HEXDIG         =  DIGIT / "A" / "B" / "C" / "D" / "E" / "F"

         HTAB           =  %x09
                                ; horizontal tab

         LF             =  %x0A
                                ; linefeed

         LWSP           =  *(WSP / CRLF WSP)
                                ; linear white space (past newline)

         OCTET          =  %x00-FF
                                ; 8 bits of data

         SP             =  %x20

         VCHAR          =  %x21-7E
                                ; visible (printing) characters

         WSP            =  SP / HTAB
                                ; white space
