def _validate_tasks_var_format(value: str) -> str:
    """Validate task variables

    Arguments:
        value {str} -- A '.' seperated string to be checked for task variable formatting

    Returns:
        str -- A string with validation error messages
    """
    add_info = ''
    parts = value.split('.')
    if len(parts) != 5:
        add_info = 'Valid tasks variables are ' \
            '"tasks.<TASKNAME>.outputs.parameters.<NAME>" and ' \
            '"tasks.<TASKNAME>.outputs.artifacts.<NAME>".'
    # check for other parts
    _, _, attr, prop, _ = parts
    if attr != 'outputs':
        add_info = 'Tasks variable can only access previous tasks "outputs".'
    elif prop not in ('parameters', 'artifacts'):
        add_info = 'Task outputs variables must be "parameters" or "artifacts".'

    return add_info