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( $_ = 'abcba' ) =~ s/b//g;        # aca
( $_ = 'abcba' ) =~ s/bc//g;       # aba
( $_ = 'abcba' ) =~ s/b(?=c)//g;   # acba

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