c:\@Work\Perl\monks>perl -wMstrict -le
"my $regex = qr{ \b f[eio]e \b }xms;
 ;;
 $_ = 'xx fee fie foe yy';
 while (m/($regex)/g) {
   my $match = $1;
   print qq{captured '$match'};
   }
"
captured 'fee'
captured 'fie'
captured 'foe'