code: def _parse_whitespace_argument(source, name):
          r
          # First match the command name itself so that we find the argument
          # *after* the command
          command_pattern = r'\\(' + name + r')(?:[\s{[%])'
          command_match = re.search(command_pattern, source)
          if command_match is not None:
              # Trim `source` so we only look after the command
              source = source[command_match.end(1):]
  
          # Find the whitespace-delimited argument itself.
          pattern = r'(?P<content>\S+)(?:[ %\t\n]+)'
          match = re.search(pattern, source)
          if match is None:
              message = (
                  'When parsing {}, did not find whitespace-delimited command '
                  'argument'
              )
              raise CommandParserError(message.format(name))
          content = match.group('content')
          content.strip()
          return content
docs: r"""Attempt to parse a single token on the first line of this source.
  
          This method is used for parsing whitespace-delimited arguments, like
          ``\input file``. The source should ideally contain `` file`` along
          with a newline character.
  
          >>> source = 'Line 1\n' r'\input test.tex' '\nLine 2'
          >>> LatexCommand._parse_whitespace_argument(source, 'input')
          'test.tex'
  
          Bracket delimited arguments (``\input{test.tex}``) are handled in
          the normal logic of `_parse_command`.