用lex识别单词
构建一个识别不同类型英语单词的简单程序。先识别词性（名词，动词等），然后再扩展到处理符合简单英语语法的多个单词的句子。
先列出要识别的一组动词：
is am are were was be being been do does did will would should can could has have had go
识别这些动词的lex程序：
%{
/*
* 例1-1 单词识别程序 ch1-02.l
* 识别单词是否是动词
*/
%}
%%
[ ]+ /*意味着空格的正闭包，忽略空白*/;
is |
am |
are |
were |
was |
be |
being |
been |
do |
does |
did |
will |
would |
should |
can |
could |
has |
have |
had |
go {printf("%s: is a verb
",yytext);}
[a-zA-Z]+ {printf("%s: is not a verb
",yytext);}
.|
{ECHO;/*通常的默认状态：输出匹配模式，复制标点或其它字符*/}
%%
int main()
{
yyin = fopen("example.txt","r");
yylex();
fclose(yyin);
}
int yywrap()
{
return 1;
}
需要的例子文件example.txt （放在相同文件夹）：
did I have fun?
程序运行cmd命令：
（1）打开lex程序文件所在目录：
如：
（2）编译命令： flex ch1-02.l 将生成lex.yy.c文件
（3）利用C++编译命令，编译运行.c文件，即可。
如下图所示：
（4）命令 ./ch1-02.exe 运行生成的exe文件，得到词法分析结果：
