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.