code: def build_pattern(log_format):
      
      if log_format == 'combined':
          log_format = LOG_FORMAT_COMBINED
      elif log_format == 'common':
          log_format = LOG_FORMAT_COMMON
      pattern = re.sub(REGEX_SPECIAL_CHARS, r'\\\1', log_format)
      pattern = re.sub(REGEX_LOG_FORMAT_VARIABLE, '(?P<\\1>.*)', pattern)
      return re.compile(pattern)
docs: Build regular expression to parse given format.
      :param log_format: format string to parse
      :return: regular expression to parse given format