%option reentrant (–reentrant) must be specified.
%option reentrant is specified in the above example
(see Reentrant Example. Had this option not been specified,
flex would have happily generated a non-reentrant scanner without
complaining. You may explicitly specify
%option noreentrant, if
you do not want a reentrant scanner, although it is not
necessary. The default is to generate a non-reentrant scanner.