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flex is a tool for generating scanners. A scanner is a
program which recognizes lexical patterns in text. The flex
program reads the given input files, or its standard input if no file
names are given, 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 by default, which defines a routine yylex().
This file can be compiled and linked with the flex runtime 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.