DFD (数据流图) 中的所有元素的集合

iterm = expr

a+b

[a|b] 或 [a, b]

(optional) → optional?

{expr} → expr*

{expr}n → expr{0, n}

m{expr}n → expr{m, n}

“atom”

n → “n” (即数字字面量不要带引号)

m..n

comment

e.g.

identifier = start_chars + {nonstart_char}
nonstart_char = [one_number | start_chars]
start_chars = chars
chars = ["a".."z" | "A".."Z" | "_"]
one_number = [0..9]