flex the fast lexical analyser generator

a flex program consists of three sections, separeated by %%/.

一个flex 程序有三部分组成，这三部分由%%分开。

 

the first section contains declarations and option settings.

 

the second section is a list of partterns and actions.

 

the third section is c code that is copied to the generated scanner.

usually small routines related to the code in the actions.