The -lfl flag links the Flex library ( libfl.a ). With version 2.5.4a, this library is simple and compatible. If you get linking errors, ensure you have installed the library or copy the source of main() and yywrap() directly into your .l file (as shown above).
While you won't typically find 2.5.4a in the default repository of a modern Linux distro (like Ubuntu 22.04 or Fedora), you might find it in archived repositories. download flex 2.5.4a
The Flex project is currently hosted on GitHub, but historical releases are archived. You can often find source tarballs for historical versions in the release notes or the "Tags" section of the repository. However, for very old versions, the source code might have been moved to a legacy archive server. The -lfl flag links the Flex library ( libfl