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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 convert(self, string, preprocess = None):
 """ Swap characters from a script to transliteration and vice versa. Optionally sanitize string by using preprocess function. :param preprocess: :param string: :return: """

        string = unicode(preprocess(string) if preprocess else string, encoding="utf-8")
        if self.regex:
            return self.regex.sub(lambda x: self.substitutes[x.group()], string).encode('utf-8')
        else:
            return string