static bool
check_input_char(char c)
{
  switch(grid_size)
    {
    case 64:
      if ((c >= 'o' && c <= 'z') ||
	  (c == '@') ||
	  (c == '*') ||
	  (c == '&'))
	break;

    case 49:
      if ((c >= 'b' && c <= 'n'))
	break;

    case 36:
      if ((c >= 'Q' && c <= 'Z') ||
	  (c == 'a'))
	break;

    case 25:
      if (c >= 'H' && c <= 'P')
	break;

    case 16:
      if (c >= 'A' && c <= 'G')
	break;

    case 9:
      if (c >= '5' && c <= '9')
	break;

    case 4:
      if(c >= '1' && c <= '4')
	break;

    case 1:
      if(c != '_')
	return false;
      break;
    }
  return true;
}