有一个在线生成工具：http://railroad.my28msec.com/rr/ui
示例1：
来自Stack Overflow
object ::= '{' ((string ':' value ) ( ',' string ':' value )*)? '}'
示例2：
包含+, -, *,括号的算术表达式描述(EBNF)
exp ::= term (addop term)*
term ::= factor ('*' factor)*
addop ::= '+'
|'-'
factor ::= '(' exp ')'
| [0-9]+
得到的Syntax/Railroad Diagram
exp:
referenced by:factor
term:
term ::=
factor ( '*'
factor )*
referenced by:exp
addop:
addop ::= '+' | '-'
referenced by:exp
factor:
referenced by:term
示例3
stmt-seq ::= stmt (';'stmt)*
stmt ::= if-stmt
| repeat-stmt
| assign-stmt
| read-stmt
| write-stmt
if-stmt ::= 'if' exp 'then' stmt-seq ('else' stmt-seq)? 'end'
repeat-stmt ::= 'repeat' stmt-seq 'util' exp
assign-stmt ::= id ':=' exp
read-stmt ::= 'read'id
write-stmt ::= 'write' exp
exp ::= simple-exp (('<'|'=') simple-exp)?
simple-exp ::= term (('+'|'-') term)*
term ::= factor(('*'|'/')factor)*
factor ::= '('exp')' | num | id
num ::=([1-9])+([0-9])*
id ::=([a-zA-Z])+
更多示例