词法分析时，常常会用到超前搜索方法。如当前待分析字符串为“a +”,当前字符为’ ’，此时，分析器倒底是将其分析为大于关系运算符还是大于等于关 系运算符呢？显然，只有知道下一个字符是什么才能下结论。于是分析器读入下一个字符’+’，这时可知应将’ ’解释为大于运算符。但此时，超前读了一个字符’+’，所以要回退一个字符，词法分析器才能正常运行。在分析标识符，无符号整数等时也有类似情况。-Lexical analysis, often use advanced search method. If the current to be analysed for "a+" string, the current characters to ", this time, is really the parser analysis for greater than or equal to relational operators greater than shut is operator? Obviously, only know the next character is what will jump to conclusions. So the parser read the next character　+ , then CPC should be　　explanation for greater than operator. But at the moment, the leading read a characters +　, so want to back a character, lexical analyzer to normal operation. On the analysis of the identifier, unsigned integer etc also has a similar situation.