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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 decode_copy_value(value):
 """ Decodes value received as part of Postgres `COPY` command. :param value: Value to decode. :type value: str :return: Either None if the value is NULL string, or the given value where escape sequences have been decoded from. :rtype: str|None """

    # Test for null values first.
    if value == POSTGRES_COPY_NULL_VALUE:
        return None

    # If there is no backslash present, there's nothing to decode.
    #
    # This early return provides a little speed-up, because it's very
    # common to not have anything to decode and then simple search for
    # backslash is faster than the regex sub below.
    if '\\' not in value:
        return value

    return DECODE_REGEX.sub(unescape_single_character, value)