Install flex-old-doc by entering the following commands in the terminal:
sudo apt update sudo apt install flex-old-doc
Description:
Documentation for an old flex (a fast lexical analyzer generator)
This package contains the HTML documentation for the old 2.5.4a version of flex, which is no longer being developed upstream. You should normally chose flex-doc, unless you have legacy lexer files that do not work with a modern flex, and need to use the old flex. The upstream source code can be found at the FTP site: http://flex.sourceforge.net/
Homepage: http://flex.sourceforge.net/
Version: 2.5.4a-10ubuntu2
Section: universe/doc