flex is a tool for generating scanners: programs which
recognized lexical patterns in text. flex 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', 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.