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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 normalize_hex(hex_value):
 """ Normalize a hexadecimal color value to 6 digits, lowercase. """

    match = HEX_COLOR_RE.match(hex_value)
    if match is None:
        raise ValueError(
            u"'{}' is not a valid hexadecimal color value.".format(hex_value)
        )
    hex_digits = match.group(1)
    if len(hex_digits) == 3:
        hex_digits = u''.join(2 * s for s in hex_digits)
    return u'#{}'.format(hex_digits.lower())