针对四则运算，写出字符集∑、相应的词法单元集合、词法单元对应的正则表达式集合 合理有效地设计符号表可存储四则运算中的各种词法单元及其属性等信息 列出关键算法的具体实现的思路-For four operations, write the character set Σ, the corresponding set of lexical units, lexical unit corresponds to a collection of regular expressions reasona