| rex field=initiatedBy.user.userPrincipalName "ex(?<GUID>\d+)d\@"
That worked thank you! Now I have a GUID field with the values I need. One last thing, how would I use the following search to convert that GUID into a username? In the example below, "235999" would be where the GUID value is inserted.
| ldapsearch domain=DEFAULT search="(&(objectClass=user)(exguid=235999))" | table name