import re
s = '''INFO:werkzeug:127.0.0.1 - - [20/Sep/2018 19:40:00] "GET /socket.io/?polling HTTP/1.1" 200 - 
INFO:engineio: Received packet MESSAGE, ["key",{"data":{"tag1":12,"tag2":13,"tag3": 14"...}}]'''

print(re.findall(r'\[([^][]*"key"[^][]*)]', s))