设计一个扫描器，该扫描器是一个子程序，其输入是源程序字符串，每调用一次识别并输出一个单词符号。为了避免超前搜索，提高运行效率，简化扫描器的设计，假设该程序设计语言中，基本字（也称关键词）不能做一般标识符用，如果基本字、标识符和常数之间没有确定的运算符或界符作间隔，则用空白作间隔。-The design of a scanner, the scanner is a subroutine, the input source string, each one call to identify and output a word symbol. In order to avoid advanced search to improve operational efficiency, simplify the design of the scanner, assuming that the programming language, the basic word (also known as keywords) can not do general identifiers, basic words, identifiers, and constants to determine operator or boundary operator for the interval with a blank interval.