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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 url(value):
 """Validate a URL. :param string value: The URL to validate :returns: The URL if valid. :raises: ValueError """

    if not url_regex.search(value):
        message = u"{0} is not a valid URL".format(value)
        if url_regex.search('http://' + value):
            message += u". Did you mean: http://{0}".format(value)
        raise ValueError(message)
    return value