Doesn't always work:

my $str = "start b2 end start b2 end start b2 end";
#only one b2 between each start and end
if ($str =~ /.*start((.*?b2.*?b2).*?)end/) {
 print "$1\n";
}
__END__
Output:

 b2 end start b2
[download]

shouldn't match when there is only one 'b2' between start and end, but it does.

IMO, the most robust solution is to use a parser (Parse::RecDescent) or multiple regexes.