code: def _validate_alias_command(alias_command):
      
      if not alias_command:
          raise CLIError(EMPTY_ALIAS_ERROR)
  
      split_command = shlex.split(alias_command)
      boundary_index = len(split_command)
      for i, subcommand in enumerate(split_command):
          if not re.match('^[a-z]', subcommand.lower()) or i > COLLISION_CHECK_LEVEL_DEPTH:
              boundary_index = i
              break
  
      # Extract possible CLI commands and validate
      command_to_validate = ' '.join(split_command[:boundary_index]).lower()
      for command in azext_alias.cached_reserved_commands:
          if re.match(r'([a-z\-]*\s)*{}($|\s)'.format(command_to_validate), command):
              return
  
      _validate_positional_arguments(shlex.split(alias_command))
docs: Check if the alias command is valid.
  
      Args:
          alias_command: The command to validate.