do_matching_exactly_no_literal_opts Subroutine

   subroutine do_matching_exactly_no_literal_opts(automaton, string, res)
      implicit none
      type(automaton_t), intent(inout) :: automaton
      character(*),      intent(in)    :: string
      logical,           intent(inout) :: res

      integer :: cur_i, dst_i ! current and destination index of DFA nodes
      integer :: ci           ! character index
      integer :: next_ci      ! next character index
      integer :: max_match    !
      character(:), allocatable :: str

      ! Initialize `cur_i` with automaton's initial index.
      cur_i = automaton%initial_index

      ! If the DFA have not been initialized, abort the program.
      if (cur_i == DFA_NOT_INIT) then
         error stop "DFA have not been initialized."
      end if

      ! If the input string is an empty string, returns a logical value
      ! indicating whether the current state is accepting or not.
      if (len(string) == 0) then
         res = automaton%dfa%nodes(cur_i)%accepted
         return
      end if

      ! Initialize counter variables.
      max_match = 0
      ci = 1
      str = char(0)//string//char(0)

      ! Loop and proceed with matching unless the current index is DFA_INVALID_INDEX.
      do while (cur_i /= DFA_INVALID_INDEX)

         ! If the current state acceptable, the value of `max_match` is updated with `i`.
         if (automaton%dfa%nodes(cur_i)%accepted) then
            max_match = ci
         end if

         if (ci > len(str)) exit

         ! Get the index of the next character and assign it to `next_ci`.
         next_ci = idxutf8(str, ci) + 1

         ! Lazy evaluation is performed by calling this procedure here.
         ! The index of destination DFA node is stored in the `dst_i` variable.
         call automaton%construct(cur_i, dst_i, str(ci:next_ci-1))

         ! If there is mismatch in the first byte of the NULL character, try again with the second byte.
         if (dst_i == DFA_INVALID_INDEX .and. ci == 1) then
            ci = 2
            next_ci = idxutf8(str, ci) + 1
            call automaton%construct(cur_i, dst_i, str(ci:next_ci-1))
         end if

         ! update counters
         cur_i = dst_i
         ci = next_ci

      end do

      ! If the maximum index of the match is one larger than length of the string,
      ! this function returns true, otherwise it returns false.
      if (max_match >= len(string)+2) then
         res = .true.
      else
         res = .false.
      end if
   end subroutine do_matching_exactly_no_literal_opts