code: def preprocess(content, options):
      # type: (Text, Optional[optparse.Values]) -> ReqFileLines
      
      lines_enum = enumerate(content.splitlines(), start=1)  # type: ReqFileLines
      lines_enum = join_lines(lines_enum)
      lines_enum = ignore_comments(lines_enum)
      lines_enum = skip_regex(lines_enum, options)
      lines_enum = expand_env_variables(lines_enum)
      return lines_enum
docs: Split, filter, and join lines, and return a line iterator
  
      :param content: the content of the requirements file
      :param options: cli options