import re


def get_acceptable_string(input_string):
    """
    Strips all unwanted characters except a-z, A-Z, 0-9, and '(). -_'
    """
    if input_string is not None:
        temp = re.sub("[^a-z0-9()-_.\s]", '', input_string, flags=re.I)
        return re.sub("\s+", " ", temp).strip()
    else:
        return None