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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def extract_version_tag_from_url(url):
 """Extract a DataONE API version tag from a MN or CN service endpoint URL. Args: url : str Service endpoint URL. E.g.: ``https://mn.example.org/path/v2/object/pid``. Returns: str : Valid version tags are currently ``v1`` or ``v2``. """

    m = re.match(r'(/|^)(v\d)(/|$)', url)
    if not m:
        return None
    return m.group(2)