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4. ABNF Definition of ABNF (ABNF的ABNF定义)

ABNF语法的ABNF表示​

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

rule           = rulename defined-as elements c-nl
                 ; 继续行如果下一行以空白开始

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

defined-as     = *c-wsp ("=" / "=/") *c-wsp
                 ; 基本规则定义和增量备选项

elements       = alternation *c-wsp

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

c-nl           = comment / CRLF
                 ; 注释或换行

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
                 ; 引号文字文本字符串
                 ; 排除 DQUOTE 本身

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

bin-val        = "b" 1*BIT
                 [ 1*("." 1*BIT) / ("-" 1*BIT) ]
                 ; 一系列串联的位值
                 ; 或单个ONEOF范围

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) ">"
                 ; 散文描述，作为最后手段

说明​

这个ABNF规范使用ABNF来定义ABNF本身。这种自引用是有效的，因为规则的含义是明确的。

注意: prose-val（散文值）用于当无法用ABNF表示语法时，作为最后的手段。它应该谨慎使用，并且应该尽可能清晰地记录。

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