gistfile1.txt

#!/usr/bin/env python3

import re
import sys

filename = sys.argv[1]
pattern = r"(https://youtu\.be/[a-zA-Z0-9_-]*)"
with open(filename, 'r') as fp:
    for line in fp.readlines():
        for match in re.findall(pattern, line):
            assert 28 == len(match)
            print(match)