Flex is available for most of the UNIX-like platforms and as the flex package from the Cygwin setup.

If GNU flex isn't already installed or available as a package for your platform, you can get it at: https://www.gnu.org/software/flex/.

After correct installation, typing at the bash command line prompt:

$ flex --version

should result in something like:

        flex version 2.5.4

However, the version string may vary.

A native Win32 lexx/flex package can be obtained from https://gnuwin32.sourceforge.net/. The installation should be straightforward.