Capturing group inside a custom grok pattern

[agosmaker]

Hello,
Let's imagine we have string
some text aaabbbccc another text
We want to extract bbb.

1. we can use "embedded" regex (https://www.elastic.co/guide/en/logstash/current/plugins-filters-grok.html Custom Patterns "option 1"
   aaa(?<superb>.*)ccc
   and get
   {
   "superb": [
   "bbb"
   ]
   }
   But it's more neatly to extract this logic to custom pattern in custom pattern file.

2. we can use custom pattern in custom pattern file ("option 2")
   BPATTERN aaa(?<superb>.*)ccc
   result:
   {
   "bvar": [
   "aaabbbccc"
   ],
   "superb": [
   "bbb"
   ]
   }

Now we have excess variable bvar.
Of course we can mutate and remove bvar, but does another method exist to not create (and remove further) bvar variable?

I tried syntax like
%{BPATTERN:}
but it doesn't allow omit variable.

[Badger]

Leave out the colon. This works

grok {
    pattern_definitions => { "BPATTERN" => "aaa(?<superb>.*)ccc" }
    match => { "message" => "%{BPATTERN}" }
}

It would work with patterns_dir too.

[agosmaker]

Thank you for the answer. I'll check but it doesn't work at least in http://grokdebug.herokuapp.com/

[agosmaker]

Yes, it works. Many thanks.
All uppercase names in http://grokdebug.herokuapp.com/ like
"YEAR": [
[
"2018"
]
]
really aren't included in ouput json by logstash. So everything is OK