flex-old-doc 2.5.4a-10ubuntu1 (arm64 binary) in ubuntu bionic

 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/