Please use the learned knowledge to write a program to find out the password hidden in the long string below. The burial point of the password conforms to the following rules:

Little turtle

1. Please write the program with the knowledge you have learned , Find out the password of the little turtle hidden in the long string below , The burial point of the password conforms to the following rules ：
   a) Each password is a single lowercase letter
   b) Each password has only three capital letters on the left and right
   Today, I just finished learning regular expressions and saw this problem , Now it feels much simpler to write in regular expressions

import re  #  Import re modular 
def fn():
    f = open('string2.txt','r')
    a = f.read()
    l = re.findall('[^A-Z]{1}[A-Z]{3}[a-z]{1}[A-Z]{3}[^A-Z]{1}',a)
    ''' Choose the letter range that meets the requirements of the topic . Small letters in the middle , There are only three large passwords on the left and right sides of the password 
     Write letters （ Explain that after the three capital letters on both sides, as long as they are non capital letters, the conditions are met ）, A list is returned '''
    for i in l:  #  Traverse the list 
        print(i[4],end=' ')  #  Print fourth letter 
fn()