def getPurKeyword(inkey):
    """
    each key is case-insensitve in lower case.
    A keyword can have a singular or plural form - so the plural-s at the end is optional.
    A keyword can be assigned as keyword by the beginning digit underscore - so it is optional too.
    A keyword in a csv-file can be characterized by the keyword delimited with the in-filed-delimiter colon.
    a sub-keyword  can be delimited
    :param inkey:
    :return:
    """
    keyPur = inkey.lower()
    if ":" in keyPur:
        keyPur = keyPur.split(":").pop()
    if "-" in keyPur:
        keyPur = keyPur.split("-").pop()
    if keyPur[0:1] == "_":
        keyPur = keyPur[1:]
    if keyPur[-1:] == "s":
        keyPur = keyPur[:-1]
    return keyPur