最近，我学习python regex，我听不懂某些表达式
# but i think it should be : a-b---c
sub_1 = re.sub("x*", "-", "abxc") # result is : -a-b-c-
# but i think it should be : a-b-x-c
sub_2 = re.sub("", "-", "abxc") # result is : -a-b-x-c-
# but i think it should be : ['a', '']
find_all_1 = re.findall("a*", "ab") # result is : ['a', '', '']
# but i think it should be : ['', '', '']
find_all_2 = re.findall("", "abc") # result is : ['', '', '', '']