data: ""{\"session_key\":\"1m5\\\/K\\\/53lyERNXHCQjJAnQ==\",\"openid\":\"o5r1Q5NzOzAJ_diaTlRuTzeFfq00\"}""

var text = JSON.parse(res.data).toString();
var pattern = /"openid":"(.*?)"/
var docId = pattern.exec(text)[1];

结果
o5r1Q5NzOzAJ_diaTlRuTzeFfq00