User.findOne({ username: {$regex: new RegExp('^' + req.body.username, 'i')} }).select('_id').exec(function (err, user) {
                if (err) {
                    res.json({  "status": 0, "message": "Something went wrong" });
                } else {
                    if (user) {
                        res.json({ "status": 0, "message": "This username is already taken." });
                    } else {
                        res.json({ "status": 1, "message": "Username is valid" });
                    }
                }
            });