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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 escape_ampersand(string):
 """ Quick convert unicode ampersand characters not associated with a numbered entity or not starting with allowed characters to a plain &amp; """

    if not string:
        return string
    start_with_match = r"(\#x(....);|lt;|gt;|amp;)"
    # The pattern below is match & that is not immediately followed by #
    string = re.sub(r"&(?!" + start_with_match + ")", '&amp;', string)
    return string