[0] Perl> $re = qr[(?=^.*this)(?=^.*that)];;
[0] Perl> $_ =~ $re and print "$_ matched" 
          for qw[thisthat thatthis thathis thisnthat thatnthis];;
thisthat matched
thatthis matched
thathis matched **
thisnthat matched
thatnthis matched