$_= "start this stop start that stop now start last stop";
print "non-greedy:\n";
print "  ", $_, $/
    for  /start(.*?)stop now/g;
print "look-ahead:\n";
print "  ", $_, $/
    for  /start((?:(?!stop).)*)stop now/g;
__END__
non-greedy:
   this stop start that
look-ahead:
   that