[{"username":"testuser_more_data","email":"email","password":"pass"},{"username":"user2","email":"email@testuser.es","password":"pass2"},{"username":"testuser","email":"testuser","password":"pass3"},{"username":"user4","email":"email4","password":"pass4"}]

db.collection.find({"$or":[{"username":{"$regex":"testuser"}},{"email":{"$regex":"testuser"}}]})