Output regex result to a field in a rule

I know this must be simple but I can’t seem to get my head around it even after going through the documentation many times. Just trying to clean up some windows events and pull some data from a field:

in this case it contains
CN=Administrator,OU=Domain Users,DC=test,DC=local

I’d like to pull the common name administrator (or whatever user is there) into its own field. I’ve got a rule to perform this:

has_field(“winlogbeat_event_id”) AND contains(to_string($message.winlogbeat_event_id), “4728”)
let pattern = “CN=([^,]+)”;
let t1 = regex(pattern,to_string($message.winlogbeat_event_data_MemberName));
set_field(“testpattern”, t1[“1”]);

I would expect this to set the second match for t1 to the field testpattern but I don’t get any result.

If I use:
set_field(“testpattern”, t1);

I’ll see events containing field testpattern filled with {“0”:“Administrator”}

Unfortunately I’m not fluent enough to figure out what I’m doing wrong. Any help would be appreciated.

did you tried set_field(“testpattern”, t1[“0”]); as the counting starts at 0 …?

Sorry, last thing I had tried and forgot to pull it out. I get the same result with:

set_field(“testpattern”, t1[“0”]);

I notice graylog is producing an indexer error that reads:
{“type”:“mapper_parsing_exception”,“reason”:“object mapping for [testpattern] tried to parse field [testpattern] as object, but found a concrete value”}

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