Description:

This is legacy version of flex, a program that generates scanners.
Scanners are programs which can recognize lexical patterns in text.
Flex takes pairs of regular expressions and C code as input and
generates a C source file as output. The output file is compiled and
linked with a library to produce an executable. The executable
searches through its input for occurrences of the regular expressions.
When a match is found, it executes the corresponding C code. Flex was
designed to work with both Yacc and Bison, and is used by many
programs as part of their build process.

You should install flex if you are going to use your system for
application development.