import re


def user_list(list_to_split=None, user_prompt='Enter a list of values: '):
    """
    Build a list from user input separated by commas.
    """
    print('\n')
    if list_to_split is not None:
        # regular expression removes any non-alpha characters, outside whitespace
        return [re.sub("[^a-zA-Z0-9 ]+", "", str(x).strip()) for x in list_to_split.split(',')]
    else:
        while True:
            values = [re.sub("[^a-zA-Z0-9 ]+", "", str(x).strip()) for x in input(user_prompt).split(',')]

            if not values[0]:
                print('No values entered.')
                print('Try again...\n')
                continue
            else:
                return values