针对四则运算，写出了字符集∑、相应的词法单元集合、词法单元对应的正则表达式集合，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-Four operations, write the character set sigma, the lexical unit set of lexical units corresponding regular expressions collection, the design of the symbol table can be stored in fo