This is interesting and worth mentioning but needs a clear caveat that the backend has no idea whether the data came from an HTML5 browser or a hacker/hackbot that bypassed the filter completely. Client side validation is for courtesy, speed, saving unnecessary server load. All data must be validated on the backend/server regardless of what the front end does.
I also think it borders or a poor practice because the best code would use the same regex on the front and back end to avoid silly bugs and the backend has much more power to offer so will be hobbled by the subset the front end can do.
