( $_ = '!a!b!c!' ) =~ s/!.!/!_!/g;         # !_!b!_!
( $_ = '!a!b!c!' ) =~ s/!.(?=!)/!_/g;      # !_!_!_!
( $_ = '!a!b!c!' ) =~ s/(?<=!).(?=!)/_/g;  # !_!_!_!