The following flex input specifies a scanner which,
whenever it encounters the string `username', will replace it
with the user's login name:
%% username printf( "%s", getlogin() );
By default, any text not matched by a flex scanner is copied to
the output, so the net effect of this scanner is to copy its input file
to its output with each occurrence of `username' expanded. In this
input, there is just one rule. `username' is the pattern and the
printf is the action. The `%%' marks the beginning of the
rules.