4. ABNF 的 ABNF 定义

本节给出 ABNF 自身的 ABNF 定义. 该语法使用 ABNF 表示法定义.

代码块中的规则是 ABNF 元语法, 用来精确定义 rulelist,rule,alternation,concatenation,repetition,group,option,char-val,num-val 和 prose-val 等构造. 这些规则名和字面量必须保持原样, 因为任何翻译都会改变语法定义本身.

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

关于 ABNF 定义的说明

ABNF 的 ABNF 定义展示了该表示法的自描述特性. 这些规则说明了如何使用 ABNF 自身形式化地定义 ABNF 语法.

关键观察:

规则使用其所描述的相同运算符来定义
该表示法是递归且自引用的
这为 ABNF 元语言 (metalanguage) 提供了形式化规范
实现解析器时应按此处的空白,注释,换行和增量替代规则处理输入, 不能只按普通文本拆分

关于 ABNF 定义的说明​

关于 ABNF 定义的说明