Extract all variables from a log format string.
    :param log_format: format string to extract
    :return: iterator over all variables in given format string

def extract_variables(log_format):
    """
    Extract all variables from a log format string.
    :param log_format: format string to extract
    :return: iterator over all variables in given format string
    """
    if log_format == 'combined':
        log_format = LOG_FORMAT_COMBINED
    for match in re.findall(REGEX_LOG_FORMAT_VARIABLE, log_format):
        yield match