The flex program 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.