I am using FLEX tool to design a simple Compiler

Just type in cmd:

sudo apt-get install flex
sudo apt-get update

Just go to this Official Site Download the complete package After Downloading,install it. Then in searchBar , Type:

edit the system environment variables

Go to Environment Variable, Under System Variables , Choose Path
Now it will pop up Edit Environment Variable. Then add Two path one is bin folder of MinGW and another is bin folder of GnuWin32

Flex file's extensions are .l or .lex run command prompt, where your .l or .lex file is located

In cmd Type :

flex filename.l
gcc lex.yy.c
./a.out

you have to create one folder to execute one .lex file
So if you have multiple .l or .lex file, have to create different different folder
why? The reason is: for each .l or .lex file , everytime a.out file will be created
Now windows doesn't support duplicate file in same folder,so it will try to merge & the previous a.out file will be lost

generally Flex programme is designed with default a.out file But if You can change the name of Object file
Then automatically the executable file name will be changed.

In CMD Type:

flex fileName.l
gcc lex.yy.c -o Anything
./Anything.exe