% perl -le '"wxy" =~ /(wx?)/;   print $1'
1:wx
% perl -le '"wxy" =~ /(wx??)/;  print $1'
1:w

% perl -le '"wxy" =~ /(wx?y)/;  print $1'
1:wxy
% perl -le '"wxy" =~ /(wx??y)/; print $1'
1:wxy