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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 grep_log(self, expr, filename='system.log', from_mark=None):
 """ Returns a list of lines matching the regular expression in parameter in the Cassandra log of this node """

        matchings = []
        pattern = re.compile(expr)
        with open(os.path.join(self.get_path(), 'logs', filename)) as f:
            if from_mark:
                f.seek(from_mark)
            for line in f:
                m = pattern.search(line)
                if m:
                    matchings.append((line, m))
        return matchings