词法分析程序的功能：

输入：所给文法的源程序字符串。

输出：二元组（syn,token或sum）构成的序列。

其中：syn为单词种别码；

token为存放的单词自身字符串；

sum为整型常数。

例如：对源程序begin x:=9: if x>9 then x:=2*x+1/3 end #的源文件，经过词法分析后输出如下序列：

(1,begin)(10,x)(18,:=)(11,9)(26, )(2,if)……

-Lexical analyzer function: Input: a grammar of the source string. Output: tuple (syn, token or sum) consisting of sequences. Where: syn is another code word kind　token string for storing word itself　sum to an integer constant. For example: the source program begin x: = 9: if x>　9 then x: = 2* x+1/3　end# of the source file after the lexical analysis following output sequence: (1, begin) (10, x) ( 18,: =) (11,9) (26,　) (2, if) ......