编译原理词法分析器的功能是利用函数之间的递归调用模拟语法树自上而下的构造过程。

改造文法：消除二义性、消除左递归、提取左因子，判断是否为LL（1）文法

-Compiler theory lexical analyzer function is to use recursive function calls between the simulation process of constructing the syntax tree from the top down. Transformation grammar: to eliminate ambiguity, to eliminate left recursion, left-factor extraction to determine whether the LL (1) grammar