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