Python re.sub() is not replacing every match

Question

I'm using Python 3 and I have two strings: abbcabb and abca. I want to remove every double occurrence of a single character. For example:

abbcabb should give c and abca should give bc.

I've tried the following regex (here):

(.)(.*?)\1

But, it gives wrong output for first string. Also, when I tried another one (here):

(.)(.*?)*?\1

But, this one again gives wrong output. What's going wrong here?

---

The python code is a print statement:

print(re.sub(r'(.)(.*?)\1', '\g<2>', s)) # s is the string

Answer 1

It can be solved without regular expression, like below

>>>''.join([i for i in s1 if s1.count(i) == 1])
'bc'
>>>''.join([i for i in s if s.count(i) == 1])
'c'

Answer 2

re.sub() doesn't perform overlapping replacements. After it replaces the first match, it starts looking after the end of the match. So when you perform the replacement on

abbcabb

it first replaces abbca with bbc. Then it replaces bb with an empty string. It doesn't go back and look for another match in bbc.

If you want that, you need to write your own loop.

while True:
    newS = re.sub(r'(.)(.*?)\1', r'\g<2>', s)
    if newS == s:
        break
    s = newS
print(newS)

DEMO