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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 matching_tokens(self, text, start=0):
 """Retrieve all token definitions matching the beginning of a text. Args: text (str):
 the text to test start (int):
 the position where matches should be searched in the string (see re.match(rx, txt, pos)) Yields: (token_class, re.Match):
 all token class whose regexp matches the text, and the related re.Match object. """

        for token_class, regexp in self._tokens:
            match = regexp.match(text, pos=start)
            if match:
                yield token_class, match