Validate a URL.

    :param string value: The URL to validate
    :returns: The URL if valid.
    :raises: ValueError

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