All the above comments have it pretty right on. The fun ? has a number of hidden meanings and can certainly be a little confusing. Overall, the ? can be very powerful when used in the right context, given any particular problem. May I recommend taking a look at the Perl regex documentation; it had proved to be a pretty good resource for questions just as these

http://www.perl.com/doc/manual/html/pod/perlre.html

Have a good one!