词法分析器的构造方法，词法分析器能够识别一些关键字（如if,else等）词法分析器能够识别一些运算符（如+,-,*,/等）词法分析器能够识别标识符

-lexical analyzer Construction, lexical analyzer can identify some keywords (eg if, else) lexical analyzer can identify some Operators (,-,*,/) lexical analyzer can identify identifier