As soon as support for the `u` flag is completed, it should be enabled for the HTML `pattern` attribute as well.
Test case: https://mathiasbynens.be/demo/pattern-u
This is already available in stable Firefox and it will ship in Chrome 53 and Opera 40.
This is now shipped in stable versions of Firefox, Chrome, and Opera. Please implement and ship this as soon as possible to avoid annoying interoperability issues.