import re


def youtube_url_validation(url):
    """
    Checks if the url is a valid youtube link.
    :param url: url to check against
    :type url: str
    :returns: None if it is not valid or the video-id if it is.
    """
    p = re.compile(
        r'(https?://)?(www\.)?(youtube|youtu|youtube-nocookie)\.(com|be)/(watch\?v=|embed/|v/|.+\?v=)?([^&=%\?]{11})')
    youtube_regex_match = re.search(p, url)
    if youtube_regex_match:
        return youtube_regex_match.group(6)
    return youtube_regex_match