# File lib/treetop/compiler/metagrammar.rb, line 2834
      def _nt_bracket_expression
        start_index = index
        if node_cache[:bracket_expression].has_key?(index)
          cached = node_cache[:bracket_expression][index]
          if cached
            cached = SyntaxNode.new(input, index...(index + 1)) if cached == true
            @index = cached.interval.end
          end
          return cached
        end

        i0, s0 = index, []
        if has_terminal?('[:', false, index)
          r1 = instantiate_node(SyntaxNode,input, index...(index + 2))
          @index += 2
        else
          terminal_parse_failure('[:')
          r1 = nil
        end
        s0 << r1
        if r1
          if has_terminal?('^', false, index)
            r3 = instantiate_node(SyntaxNode,input, index...(index + 1))
            @index += 1
          else
            terminal_parse_failure('^')
            r3 = nil
          end
          if r3
            r2 = r3
          else
            r2 = instantiate_node(SyntaxNode,input, index...index)
          end
          s0 << r2
          if r2
            i4 = index
            if has_terminal?('alnum', false, index)
              r5 = instantiate_node(SyntaxNode,input, index...(index + 5))
              @index += 5
            else
              terminal_parse_failure('alnum')
              r5 = nil
            end
            if r5
              r4 = r5
            else
              if has_terminal?('alpha', false, index)
                r6 = instantiate_node(SyntaxNode,input, index...(index + 5))
                @index += 5
              else
                terminal_parse_failure('alpha')
                r6 = nil
              end
              if r6
                r4 = r6
              else
                if has_terminal?('blank', false, index)
                  r7 = instantiate_node(SyntaxNode,input, index...(index + 5))
                  @index += 5
                else
                  terminal_parse_failure('blank')
                  r7 = nil
                end
                if r7
                  r4 = r7
                else
                  if has_terminal?('cntrl', false, index)
                    r8 = instantiate_node(SyntaxNode,input, index...(index + 5))
                    @index += 5
                  else
                    terminal_parse_failure('cntrl')
                    r8 = nil
                  end
                  if r8
                    r4 = r8
                  else
                    if has_terminal?('digit', false, index)
                      r9 = instantiate_node(SyntaxNode,input, index...(index + 5))
                      @index += 5
                    else
                      terminal_parse_failure('digit')
                      r9 = nil
                    end
                    if r9
                      r4 = r9
                    else
                      if has_terminal?('graph', false, index)
                        r10 = instantiate_node(SyntaxNode,input, index...(index + 5))
                        @index += 5
                      else
                        terminal_parse_failure('graph')
                        r10 = nil
                      end
                      if r10
                        r4 = r10
                      else
                        if has_terminal?('lower', false, index)
                          r11 = instantiate_node(SyntaxNode,input, index...(index + 5))
                          @index += 5
                        else
                          terminal_parse_failure('lower')
                          r11 = nil
                        end
                        if r11
                          r4 = r11
                        else
                          if has_terminal?('print', false, index)
                            r12 = instantiate_node(SyntaxNode,input, index...(index + 5))
                            @index += 5
                          else
                            terminal_parse_failure('print')
                            r12 = nil
                          end
                          if r12
                            r4 = r12
                          else
                            if has_terminal?('punct', false, index)
                              r13 = instantiate_node(SyntaxNode,input, index...(index + 5))
                              @index += 5
                            else
                              terminal_parse_failure('punct')
                              r13 = nil
                            end
                            if r13
                              r4 = r13
                            else
                              if has_terminal?('space', false, index)
                                r14 = instantiate_node(SyntaxNode,input, index...(index + 5))
                                @index += 5
                              else
                                terminal_parse_failure('space')
                                r14 = nil
                              end
                              if r14
                                r4 = r14
                              else
                                if has_terminal?('upper', false, index)
                                  r15 = instantiate_node(SyntaxNode,input, index...(index + 5))
                                  @index += 5
                                else
                                  terminal_parse_failure('upper')
                                  r15 = nil
                                end
                                if r15
                                  r4 = r15
                                else
                                  if has_terminal?('xdigit', false, index)
                                    r16 = instantiate_node(SyntaxNode,input, index...(index + 6))
                                    @index += 6
                                  else
                                    terminal_parse_failure('xdigit')
                                    r16 = nil
                                  end
                                  if r16
                                    r4 = r16
                                  else
                                    if has_terminal?('word', false, index)
                                      r17 = instantiate_node(SyntaxNode,input, index...(index + 4))
                                      @index += 4
                                    else
                                      terminal_parse_failure('word')
                                      r17 = nil
                                    end
                                    if r17
                                      r4 = r17
                                    else
                                      @index = i4
                                      r4 = nil
                                    end
                                  end
                                end
                              end
                            end
                          end
                        end
                      end
                    end
                  end
                end
              end
            end
            s0 << r4
            if r4
              if has_terminal?(':]', false, index)
                r18 = instantiate_node(SyntaxNode,input, index...(index + 2))
                @index += 2
              else
                terminal_parse_failure(':]')
                r18 = nil
              end
              s0 << r18
            end
          end
        end
        if s0.last
          r0 = instantiate_node(SyntaxNode,input, i0...index, s0)
          r0.extend(BracketExpression0)
        else
          @index = i0
          r0 = nil
        end

        node_cache[:bracket_expression][start_index] = r0

        r0
      end