此为词法分析程序，词法分析要求：简单词法文法：

<关键字>->main|int|char|string|while|do|if|else|for



<标识符>->L|<标识符>L|<标识符>D

<整数>->D|<整数>D

<运算符>->+|-|*|/|=|<|>|<=|>=|<>|= =

<界符>->( | ) | [ | ] | { |}

具体内容见报告-This is the lexical analysis process, lexical analysis of requirements: a simple lexical grammar: < keyword> ->　main | int | char | string | while | do | if | else | for < identifier> ->　L | < identifier>　L | < identifier>　D < integer> ->　D | < integer>　D < operator >->+|-|*|/|=|< |> |< =|> =|< > |= = < sector Fu> ->　(|) | [|] | (|)　the specific contents of the report