code: def _parse_track_id(self,sdp):
          '''从sdp中解析trackID=2形式的字符串'''
          m = re.search(r'a=control:(?P<trackid>[\w=\d]+)',sdp,re.S)
          return (m and m.group('trackid')) or 'f _parse_track_id(self,sdp):
          '''从sdp中解析trackID=2形式的字符串'''
          m = re.search(r'a=control:(?P<trackid>[\w=\d]+)',sdp,re.S)
          return (m and m.group('trackid')) or ''
docs: 从sdp中解析trackID=2形式的字符串