tree_graph__char_class Subroutine

   pure subroutine tree_graph__char_class(self)
      use :: forgex_utf8_m, only: idxutf8, len_utf8, count_token, ichar_utf8
      use :: forgex_cube_m, only: cube_t, assignment(=)
      use :: forgex_enums_m
      implicit none
      class(tree_t), intent(inout) :: self

      ! type(segment_t), allocatable :: seglist(:)
      type(cube_t) :: cube
      character(:), allocatable :: buf
      type(tree_node_t) :: node

      integer :: siz, ie
      logical :: is_inverted, backslashed
      character(:), allocatable :: prev, curr

      siz = 0

      call self%tape%get_token(class_flag=.true.)

      ! The variable buf stores the string representing the character class.
      buf = ''
      prev = ''
      curr = ''
      backslashed = .false.
      outer: do while (self%tape%current_token /= tk_rsbracket)
         prev = curr
         if (self%tape%current_token == tk_end) then
            return
         end if

         ie = idxutf8(self%tape%token_char, 1)
         curr = self%tape%token_char(1:ie)
         buf = buf//curr

         if (self%tape%current_token == tk_backslash .and. .not. backslashed) then
            backslashed = .true.
         else
            backslashed = .false.
         end if

         call self%tape%get_token(class_flag=.true.)
         
         ! for an escaped right square bracket 
         if (self%tape%current_token == tk_rsbracket .and. backslashed) then
            ie = idxutf8(self%tape%token_char, 1)
            curr = self%tape%token_char(1:ie)
            buf = buf//curr
            call self%tape%get_token(class_flag=.true.)
         end if

      end do outer

      ! If the character class pattern is empty, return false. 
      if (len(buf) == 0) then
         self%code = SYNTAX_ERR_EMPTY_CHARACTER_CLASS
         self%is_valid = .false.
         return
      end if

      ! Handling a negative class case.
      is_inverted = .false.
      if (buf(1:1) == SYMBOL_CRET) then
         is_inverted = .true.
         buf = buf(2:len(buf))  ! May this assignment be a problem?
      end if

      ! The variable siz stores the length of buf.
      siz = len_utf8(buf)

      if (siz < 1) then
         self%code = SYNTAX_ERR_EMPTY_CHARACTER_CLASS
         self%is_valid = .false.
         return
      end if

      call interpret_class_string(buf, cube, self%is_valid, self%code)

      if (.not. self%is_valid) then
         return
      end if

      ! the seglist array have been allocated near the L362.
      if (is_inverted) then
         call cube%invert()
      end if

      node = make_tree_node(op_char)

      ! Manually cube_t copy
      if (.not. cube%is_switched_to_bmp)then 
         node%c%ascii = cube%ascii
      else
         call node%c%switch_bmp()
         node%c%bmp%b(:) = cube%bmp%b(:)
      end if

      node%c%single_flag = cube%single_flag
      if (allocated(cube%sps)) node%c%sps = cube%sps(:)
      if (cube%is_flagged_epsilon()) call node%c%flag_epsilon()

      call self%register_connector(node, terminal, terminal)

   end subroutine tree_graph__char_class