When searching XML files, I seem to be able to match only on values and not tags. Removing the XML declaration (e.g. <?xml version="1.0" encoding="UTF-8"?>) from the file allows the same regex to match on tags.
Under the Monitors Advanced settings, check for the following;
There is the option ContentExtraction.MarkupAsText – turning that to “on” should allow us to match within tags, otherwise they are ignored.