flex - 2.6.4-8.2 main

Flex is a tool for generating scanners: programs which recognized lexical
patterns in text. It reads the given input files for a description of a
scanner to generate. The description is in the form of pairs of regular
expressions and C code, called rules. Flex generates as output a C source
file, lex.yy.c, which defines a routine yylex(). This file is compiled
and linked with the -lfl library to produce an executable. When the
executable is run, it analyzes its input for occurrences of the regular
expressions. Whenever it finds one, it executes the corresponding C code.