c:\@Work\Perl\monks>perl -wMstrict -le
"use Benchmark qw(cmpthese);
 ;;
 print 'Perl version: ', $];
 ;;
 my $s = 'Aid bears out ';
 $s = $s x 10_000_000;
 print 'length: ', length $s;
 ;;
 cmpthese(-1, {
   '/i'       => sub { $s =~ m{ (?i) [aeiou]{2}          }xmsg },
   '[Aa]{2}'  => sub { $s =~ m{ [AaEeIiOoUu]{2}          }xmsg },
   '[Aa][Aa]' => sub { $s =~ m{ [AaEeIiOoUu][AaEeIiOoUu] }xmsg },
   });
"
Perl version: 5.014004
length: 140000000
              Rate       /i  [Aa]{2} [Aa][Aa]
/i       3276565/s       --      -8%      -9%
[Aa]{2}  3558515/s       9%       --      -1%
[Aa][Aa] 3600879/s      10%       1%       --