import re
regex = r"^BEGIN\b(.*(?:\r?\n(?!(?:BEGIN|END)\b).*)*)\r?\nEND"
test_str = ("Some_text Some_text 1 2 3\n"
"BEGIN Some_text Some_text\n"
"44 76 1321\n"
"Some_text Some_text\n"
"END Some_text\n"
"Some_Text Some_text 1 4 5\n")
print(re.findall(regex, test_str, re.MULTILINE))
aW1wb3J0IHJlCgpyZWdleCA9IHIiXkJFR0lOXGIoLiooPzpccj9cbig/ISg/OkJFR0lOfEVORClcYikuKikqKVxyP1xuRU5EIgoKdGVzdF9zdHIgPSAoIlNvbWVfdGV4dCBTb21lX3RleHQgMSAyIDNcbiIKCSJCRUdJTiBTb21lX3RleHQgU29tZV90ZXh0XG4iCgkiNDQgNzYgMTMyMVxuIgoJIlNvbWVfdGV4dCBTb21lX3RleHRcbiIKCSJFTkQgU29tZV90ZXh0XG4iCgkiU29tZV9UZXh0IFNvbWVfdGV4dCAxIDQgNVxuIikKCQpwcmludChyZS5maW5kYWxsKHJlZ2V4LCB0ZXN0X3N0ciwgcmUuTVVMVElMSU5FKSk=