There is Two way to use it into your windows system. i) Manually install flex and use it from cmd. ii) Install IDE for flex/bison/Yacc WAY-1> 1-Go to given URL and download the zip file. http://sourceforge.net/projects/flex/ 2-Extract it and run Bison.exe and then Flex.exe 3-Ensure that they are in same directory normally called "GnuWin32" 4-make path for flex %default installed dir%\GnuWin32\bin\ 5-Now it is ready to use. to check type "flex --version". WAY-2> 1- Go to the provided url: FLEX IDE LINK 2- Download the exe approx. 30-32 MB 3- Install the exe setup. after installing start it, it will look like this 4-Now you are ready to ...