pattern= doesn't seem to work as expected

See jsfiddle at http://jsfiddle.net/d5w4jpxq/71/ :

``

Still allows data that doesn't start with a hash to be entered. The docs mention HTML5 pattern support - should we use `pattern` to limit the input or should we do this via `change` events?

Start by reproducing the behavior in the linked JSFiddle, then inspect the project's documented HTML5 pattern-support path and its input or change handling. Done means the intended pattern behavior is decided and the reproduction no longer accepts input that does not start with #.

Written by the indexing model from the issue text.