4. ABNF 的 ABNF 定义

注意:

该语法对规则的排版要求相对严格. 因此, 规范中所包含的规则集版本可能需要预处理, 以确保它能够被 ABNF 解析器解释.
该语法使用附录 B 中提供的规则.

      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

      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