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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 css_unescape(content, string=False):
 """ Unescape CSS value. Strings allow for spanning the value on multiple strings by escaping a new line. """


    def replace(m):
        """Replace with the appropriate substitute."""

        if m.group(1):
            codepoint = int(m.group(1)[1:], 16)
            if codepoint == 0:
                codepoint = UNICODE_REPLACEMENT_CHAR
            value = util.uchr(codepoint)
        elif m.group(2):
            value = m.group(2)[1:]
        elif m.group(3):
            value = '\ufffd'
        else:
            value = ''

        return value

    return (RE_CSS_ESC if not string else RE_CSS_STR_ESC).sub(replace, content)