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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 _parse_arg(self, arg):
 """Parse an argument."""

        match = self.SPECIAL_RE.search(arg)
        if match:
            raise KatcpSyntaxError("Unescaped special %r." % (match.group(),))
        return self.UNESCAPE_RE.sub(self._unescape_match, arg)