话说，我对当初第一次接触json这事儿印象十分深刻，原因就在于当时在json.org看到的那些-不知其名的-图：

之所以又念叨起它来，源自这两天在看LDIF 的RFC——The LDAP Data Interchange Format (LDIF) - Technical Specification。其中<Formal Syntax Definition of LDIF>这个section里用纯文字（by ABNF）描述了LDIF的语法。描述固然精确(不精确也不能成为RFC了)，但很显然不如json的这种直观。

实在看得太晕，就想找找有没有用相似形式来表达LDIF语法的现成的东西。

Google还是让我没花什么力气地找到了答案，而且还是一箭双雕。在stackoverflow里有一个这样的帖子 非但告诉了我这个图的学名——Railroad Diagram (aka. Syntax Diagram) ，还告诉了我有这么一个在线网站可以生成它！awesome!!!

这个在线工具支持输入EBNF的文本语法描述，输出railroad diagram。我稍微花了点时间将LDIF rfc里的ABNF改成了EBNF，于是就有了这个很直观易懂且支持跳转操作的railroad diagram(!!!点我查看!!!)
附我改写的ENBF:

[code title=”EBNF”]
ldif-file ::= ldif-content | ldif-changes
ldif-content ::= version-spec (SEP+ ldif-attrval-record)+
ldif-changes ::= version-spec (SEP ldif-change-record)+
ldif-attrval-record ::= dn-spec SEP attrval-spec+
ldif-change-record ::= dn-spec SEP (control)* changerecord

// version-number MUST be “1” for the LDIF format described in this document.
version-spec ::= “version:” FILL version-number

version-number ::= DIGIT+

dn-spec ::= “dn:” (FILL distinguishedName

”:” FILL base64-distinguishedName)

// distinguished name, as defined in [3]
distinguishedName ::= SAFE-STRING

//a distinguishedName which has been base64 encoded (see note 10, below)
base64-distinguishedName ::= BASE64-UTF8-STRING

//a relative distinguished name, defined as in [3]
rdn ::= SAFE-STRING

//an rdn which has been base64 encoded (see note 10, below)
base64-rdn ::= BASE64-UTF8-STRING

//(See note 9, below)
control ::= “control:” FILL ldap-oid (SPACE+ (“true” | “false”))? (value-spec)? SEP

//An LDAPOID, as defined in [4]
ldap-oid ::= DIGIT+ (“.” DIGIT+)?

attrval-spec ::= AttributeDescription value-spec SEP

//See notes 7 and 8, below
value-spec ::= “:” ( FILL (SAFE-STRING)? | “:” FILL (BASE64-STRING) | “<” FILL URL)

url ::= ‘a Uniform Resource Locator, as defined in [6]’

//Definition taken from [4]
AttributeDescription ::= AttributeType (“;” options)?

AttributeType ::= ldap-oid

(ALPHA (attr-type-chars)*)

options ::= option

(option “;” options)

option ::= opt-char+

attr-type-chars ::= ALPHA

DIGIT

”-“

opt-char ::= attr-type-chars

changerecord ::= “changetype:” FILL (change-add

change-delete

change-modify

change-moddn)

change-add ::= “add” SEP attrval-spec+

change-delete ::= “delete” SEP

change-moddn ::= (“modrdn”

“moddn”) SEP “newrdn:” ( FILL rdn

”:” FILL base64-rdn) SEP “deleteoldrdn:” FILL (“0”

“1”) SEP (“newsuperior:”( FILL distinguishedName

”:” FILL base64-distinguishedName) SEP)?

change-modify ::= “modify” SEP mod-spec*

mod-spec ::= (“add:”

“delete:”

“replace:”) FILL AttributeDescription SEP attrval-spec* “-“ SEP

SPACE ::= #x20

FILL ::= SPACE*

SEP ::= (CR LF

LF)

CR ::= #x0D

LF ::= #x0A

ALPHA ::= [A-Z]

[a-z]

DIGIT ::= [0-9]

UTF8-1 ::= [#x80-#xBF]

UTF8-2 ::= [#xC0-#xDF] UTF8-1

UTF8-3 ::= [#xE0-#xEF] UTF8-1 UTF8-1

UTF8-4 ::= [#xF0-#xF7] UTF8-1 UTF8-1 UTF8-1

UTF8-5 ::= [#xF8-#xFB] UTF8-1 UTF8-1 UTF8-1 UTF8-1

UTF8-6 ::= [#xFC-#xFD] UTF8-1 UTF8-1 UTF8-1 UTF8-1 UTF8-1

/* any value <= 127 decimal except NUL, LF, and CR*/
SAFE-CHAR ::= [#x01-#x09] | [#x0B-#x0C] | [#x0E-#x7F]

/* any value <= 127 except NUL, LF, CR, SPACE, colon (“:”, ASCII 58 decimal) and less-than (“<” , ASCII 60 decimal)*/
SAFE-INIT-CHAR ::= [#x01-#x09] | [#x0B-#x0C] | [#x0E-#x1F] | [#x21-#x39] | #x3B | [#x3D-#x7F]

SAFE-STRING ::= (SAFE-INIT-CHAR SAFE-CHAR*)?

UTF8-CHAR ::= SAFE-CHAR

UTF8-2

UTF8-3

UTF8-4

UTF8-5

UTF8-6

UTF8-STRING ::= UTF8-CHAR*

/*MUST be the base64 encoding of a UTF8-STRING*/
BASE64-UTF8-STRING ::= BASE64-STRING

/* +, /, 0-9, =, A-Z, and a-z as specified in [5] */
BASE64-CHAR ::= ‘+’ | ‘|’ | [0-9] | ‘=’ | [A-Z] | [a-z]

BASE64-STRING ::= (BASE64-CHAR*)?
[/code]

btw: 同样是在stackoverflow的那个帖子里，有人问Douglas Crockford是用什么工具画的json的railroad diagram的，得到的回答是Microsoft Visio.

试想如果json.org一打开直接是一篇ABNF的描述，80%人的下一步动作估计就是关窗口了。

谁说外表不重要的？