import re
def ipv4_address(ip):
        return (bool(re.match(
        '^(([0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5]).){3}
        ([0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])$', ip))
        if True else False)

print(ipv4_address("127.0.0.1n"))

A(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3}Z

import re

octet = r'(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])'
rx = re.compile(r"A{0}(?:.{0}){{3}}Z".format(octet))

def ipv4_address(ip):
        return bool(rx.match(ip))

strs = ["127.0.0.1n","127.0.0.2"]
for s in strs:
    print("'{}' => {}".format(s, ipv4_address(s)))

'127.0.0.1
' => False
'127.0.0.2' => True