I don't see the set of events.
Anyway, to extract the last word od an event and put it in a field, you could use a regex like this:
| rex "\s+(?<my_field>\w+)$"
that you can test at https://regex101.com/r/hofrdl/1 .
Assuming you just want the last word in each event, this should work fine:
If you want a more restrictive match, looking for only "begin" or "end", then this should work:
Hope that helps!
Edit: Used https://regexr.com/ to test/generate regex.