def METHOD_NAME(tag):
    """
    Normalize a tag for a tag expression:
      * strip whitespace
      * strip '@' char
      * convert '~' (tilde) into '-' (minus sign)
    :param tag:  Tag (as string).
    :return: Normalized tag (as string).
    """
    tag = tag.strip()
    if tag.startswith('@'):
        tag = tag[1:]
    elif tag.startswith('-@') or tag.startswith('~@'):
        tag = '-' + tag[2:]
    elif tag.startswith('~'):
        tag = '-' + tag[1:]
    return tag