Lex是美国Bell实验室用C语言研制的一个词法分析程序自生成工具。它的基本原理就是使用正则表达式扫描匹配文本，并为每一个匹配模式定义一些操作，当用C语言作宿主语言时，这些操作都由C语言实现。一种匹配的正则表达式可能会包含相关的动作。这一动作可能还包括返回一个标记。当 Lex 接收到文件或文本形式的输入时，它试图将文本与正则表达式进行匹配。它一次读入一个输入字符，直到找到一个匹配的模式。如果能够找到一个匹配的模式，Lex 就执行相关的动作（可能包括返回一个标记）。-Lex Bell Laboratories of the United States with C language developed by a lexical analysis tool generated from the process. Its fundamental principle is to use a regular expression matching text scanning, and for every definition of a matching model for operation, when the C language of the host language, these operators by C language. A match of which is likely to contain expressions related to the moves. This action may also include the return of a marker. When Lex reception of the document or text input, it is trying to text and is a regular expression matching. It read an input into the characters, until they found a matching pattern. If they can find a matching model, Lex on the implementation of the action (which may include a return of markings).