How to sort list of String without considering Special characters and with case insen

>>> list1=['test1_two','testOne','testTwo','test_one']
>>> d={}
>>> for i in list1: d[i.replace('_','').lower()]=i
...
>>> list2=d.keys()
>>> list2.sort()
>>> list3=[d[i] for i in list2]
>>> list3
['test1_two', 'test_one', 'testTwo']
>>>

class super_duper:

    '''
        f is a transformation function
        retain is a list of objects to keep

        $ python -m doctest thisFile.py

        >>> import string
        >>> list1 = ['test1_two','testOne','testTwo','test_one']
        >>> list1.sort(key=super_duper(retain=string.lowercase,f=string.lower))
        >>> list1 == 'testOne test_one test1_two testTwo'.split()
        True
    '''

    def __init__(self,retain,f):
        self.retain = set(retain)
        self.f = f

    def __call__(self,o):
        return ''.join(c for c in self.f(o) if c in self.retain)