import re


def parse_path_info(path_info):
    """
    Parse path info formatted like a/c/f where c and f are optional
    and a leading / accepted.
    Return tuple (a, c, f). If invalid path_info a is set to None.
    If c or f are omitted they are set to None.
    """

    mo = re.match(r'^/?(?P<a>\w+)(/(?P<c>\w+)(/(?P<f>\w+))?)?$',
                  path_info)
    if mo:
        return (mo.group('a'), mo.group('c'), mo.group('f'))
    else:
        return (None, None, None)