I made a XML tagmatcher plugin and released in in Graylog marketplace. The idea is to make matching XML files like
<Event> <EventTime>00:00:00</EventTime> </Event>
easy and remove the horror of backtracking that is present in regex extractors. This matcher is guaranteed to work in O(n) time, where n is the length of the source string.
I had a couple of technical difficulties in publishing the plugin: first; maven did not include the jar file to the release. Also, for some reason the marketplace does not show a direct download link to the release, but clicking on the download link in the README section goes to the right place.
The plugin can be downloaded here: