>perl -wMstrict -le
"my %rx = (
   FOO => qr{ (?i) foo (?!       bar) }xms,
   BAR => qr{ (?i) bar (?= (?-i) foo) }xms,
   );
 ;;
 my $s = 'xxx FooBar fOO BaRFoO bArfoo bar yyy';
 for my $type (keys %rx) {
   print qq{parsing $type-ish stuff with $rx{$type}};
   while ($s =~ m{ ($rx{$type}) }xmsg) {
     print qq{  found a '$1'};
     }
   }
"
parsing BAR-ish stuff with (?msx-i: (?i) bar (?= (?-i) foo) )
  found a 'bAr'
parsing FOO-ish stuff with (?msx-i: (?i) foo (?!       bar) )
  found a 'fOO'
  found a 'FoO'
  found a 'foo'