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

Question

Please let me know how to sort the list of String in either ascending / descending order without considering special characters and case.

ex:

list1=['test1_two','testOne','testTwo','test_one']

Applying the list.sort /sorted method results in sorted list

['test1_two', 'testOne', 'testTwo', 'test_one']

but the without considering the special characters and case it should be

['testOne','test_one', 'test1_two','testTwo'] OR 
['test_one','testOne','testTwo', 'test1_two' ]

list.sort /sorted method sorts based on the ascii value of the characters but Please let me knwo how do i achieve my expected one

Answer 1

If by special chars you mean "everything that is not letters":

sorted(list1, key=lambda x: re.sub('[^A-Za-z]+', '', x).lower())

Answer 2

It depends on what you mean by "special" characters - but whatever your definition, the easiest way to do this is to define a key function.

If you only care about letters:

from string import letters, digits

def alpha_key(text):
    """Return a key based on letters in `text`."""
    return [c.lower() for c in text if c in letters]

>>> sorted(list1, key=alpha_key)
['testOne', 'test_one', 'test1_two', 'testTwo']

If you care about digits as well:

def alphanumeric_key(text):
    """Return a key based on letters and digits in `text`."""
    return [c.lower() for c in text if c in letters + digits]

>>> sorted(list1, key=alphanumeric_key)
['test1_two', 'testOne', 'test_one', 'testTwo']

If you care about letters and digits and you want digits to sort after letters (which looks like it might be the case from your example output):

def alphanum_swap_key(text):
    """Return a key based on letters and digits, with digits after letters."""
    return [ord(c.lower()) % 75 for c in text if c in letters + digits]

>>> sorted(list1, key=alphanum_swap_key)
['testOne', 'test_one', 'testTwo', 'test1_two']

This last one takes advantage of the fact that "z" comes 74 places after "0" in ASCII.