第９页
--------------------------------------------------------------------------------------------------------
3.2. HEADER FIELD DEFINITIONS
These rules show a field meta-syntax, without regard for the
particular type or internal syntax. Their purpose is to permit
detection of fields; also, they present to higher-level parsers
an image of each field as fitting on one line.
field = field-name ":" [ field-body ] CRLF
field-name = 1*<any CHAR, excluding CTLs, SPACE, and ":">
field-body = field-body-contents
[CRLF LWSP-char field-body]
field-body-contents =
<the ASCII characters making up the field-body, as
defined in the following sections, and consisting
of combinations of atom, quoted-string, and
specials tokens, or else consisting of texts>
--------------------------------------------------------------------------------------------------------
3.2. 消息头域定义
这些规则是消息头域的元语法定义, 它不是为了定义特殊的类型或者某个内部语法
(是通用的定义), 此语法定义的目的是为了在一行里方便的找到消息头的域, 并且把消息
的各部分传递给更高级的解析器.
域 = 域名 ":" [ 域主体 ] 回车换行
域名 = 1*<任意字符, 除了 控制字符, 空格, 和 ":">
域主体 = 域内容
[回车换行 分割符 域主体]
域内容 =
<ASCII字符组成了域主体，就如下面章节的定义，
由词，被引串，特殊字符和其他的文本联合组成域内容>
--------------------------------------------------------------------------------------------------------
第10页
--------------------------------------------------------------------------------------------------------
3.3. LEXICAL TOKENS
The following rules are used to define an underlying lexical
analyzer, which feeds tokens to higher level parsers. See the
ANSI references, in the Bibliography.
; ( Octal, Decimal.)
CHAR = <any ASCII character> ; ( 0-177, 0.-127.)
ALPHA = <any ASCII alphabetic character>
; (101-132, 65.- 90.)
; (141-172, 97.-122.)
DIGIT = <any ASCII decimal digit> ; ( 60- 71, 48.- 57.)
CTL = <any ASCII control ; ( 0- 37, 0.- 31.)
character and DEL> ; ( 177, 127.)
CR = <ASCII CR, carriage return> ; ( 15, 13.)
LF = <ASCII LF, linefeed> ; ( 12, 10.)
SPACE = <ASCII SP, space> ; ( 40, 32.)
HTAB = <ASCII HT, horizontal-tab> ; ( 11, 9.)
<"> = <ASCII quote mark> ; ( 42, 34.)
CRLF = CR LF
LWSP-char = SPACE / HTAB ; semantics = SPACE
linear-white-space = 1*([CRLF] LWSP-char) ; semantics = SPACE
; CRLF => folding
specials = "(" / ")" / "<" / ">" / "@" ; Must be in quoted-
/ "," / ";" / ":" / "/" / <"> ; string, to use
/ "." / "[" / "]" ; within a word.
delimiters = specials / linear-white-space / comment
text = <any CHAR, including bare ; => atoms, specials,
CR & bare LF, but NOT ; comments and
including CRLF> ; quoted-strings are
; NOT recognized.
atom = 1*<any CHAR except specials, SPACE and CTLs>
quoted-string = <"> *(qtext/quoted-pair) <">; Regular qtext or
; quoted chars.
qtext = <any CHAR excepting <">, ; => may be folded
"/" & CR, and including
linear-white-space>
domain-literal = "[" *(dtext / quoted-pair) "]"
--------------------------------------------------------------------------------------------------------
3.3. 词定义
下面的规则用来定义基本的词法分析规则，
本规则是为了把词分析出来送到更高级的解析器,参看参考数目里的ANSI引用
; ( 八进制，十进制 )
字符 = <任意 ASCII 字符> ; ( 0-177, 0.-127.)
字母 = <任意 ASCII 字母> ; (101-132, 65.- 90.)
; (141-172, 97.-122.)
数字 = <任意 ASCII 十进制数字> ; ( 60- 71, 48.- 57.)
控制字符 = <任意 ASCII 控制字符 ; ( 0- 37, 0.- 31.)
和删除字符DEL> ; ( 177, 127.)
回车 = <ASCII CR, 回车> ; ( 15, 13.)
换行 = <ASCII LF, 换行> ; ( 12, 10.)
空格 = <ASCII SP, 空格> ; ( 40, 32.)
制表符 = <ASCII HT, 制表符> ; ( 11, 9.)
<"> = <ASCII 引号> ; ( 42, 34.)
回车换行 = 回车 换行
间隔符 = 空格 / 制表符 ; 语义上，可以把间隔符看成空格
折行符 = 1*([回车换行] 间隔符) ; 语义上，可以把间隔符看成空格
; 回车换行 => 折行
特殊字符 = "(" / ")" / "<" / ">" / "@" ; 为了在一个词里使用，必须用引号
/ "," / ";" / ":" / "/" / <"> ; 引起来
/ "." / "[" / "]" ;
分隔符 = 特殊字符 / 折行符 / 注释
文本 = <任意字符, 包括单独的回车， ; => 词, 特殊字符,
单独的换行，但是不能包括 ; 注释和被引串
回车换行> ; 不是文本
;
词 = 1*<任意字符 除了特殊字符，空格，控制字符>
被引串 = <"> *(被引文本/转义字符) <"> ; 通常的被引文本或者
; 被引字符
被引文本 = <任意字符，除了<">,"/", ; => 可能会折行
回车, 还包含折行符>
域名 = "[" *(域名文本 / 转义字符) "]"
--------------------------------------------------------------------------------------------------------