python中返回字符串中的重复字段

麻烦各位大佬看看如何用helper function和recursion解决改题，不要用for循环，还没有学过，谢谢

import re

def longest_contained_square(s):
    rs = re.findall(r'((.+)\2+)',s)
    if len(rs)==0:
        return ""
    else:
        return rs[0][0]

print(longest_contained_square("crocsqxbanananan"))
print(longest_contained_square("monkey"))