Install Flex by running the following commands:
./configure --prefix=/usr &&
make &&
make install
The Flex package contains the flex program
Flex is a tool for generating programs which regognize patterns in text. Pattern recognition is very useful in many applications. You set up rules what to look for and flex will make a program that looks for those patterns. The reason people use flex is that it is much easier to set up rules for what to look for than to write the actual program that finds the text.