sed提取json中有用字符串

 
mytest='"summary":{"day":{"burstTotal":0,"dropTotal":135764,"flowTotal":69449},"input":{"spanCount":900,"spanInterval":10},"spanNow":{"burstTotal":0,"dropTotal":1,"flowTotal":4,"now":1548691410},"spans"'
echo $mytest| sed 's/\(.*\)"day":{"burstTotal":\([[:alnum:]]*\),"dropTotal":\([[:alnum:]]*\),"flowTotal":\([[:alnum:]]*\).*/\4/' && echo ":flow121"