"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.