Unison ignore all files in path except for html files

Question

I want to be able to ignore all files under any directory named build except for .html files.

So for example:

Ignored:

MyProject/build/manifest.xml
MyProject/Package1/build/code.java

Not Ignored

MyProject/src/manifest.xml
MyProject/build/index.html

Currently I have tried these rules (and many besides):

ignore = Regex ^.*(build)+.+\.+.{1,5}$
ignorenot = Regex ^.*(build)+.*(\.html)$

However they don't work; how can I accomplish this?