fluentd - When a specific value can be used as a key but not recoded

Question

In the example below, i can use $ ["kubernetes"]["aaa"]["bbb"] as the key value,

but It cannot be used as a tag value. $ {recode ["kubernetes"]["aaa"]["bbb"]} also doesn't work.

How can I use it as a tag ??

Why is it so different?

  <match **>
    @type rewrite_tag_filter
    <rule>
      key $["kubernetes"]["aaa"]["bbb"]
      pattern ".*"
      tag ${recode["kubernetes"]["aaa"]["bbb"]}
    </rule>
   </match>

Answer 1

If you put your regex pattern within grouping brackets () then entire matching text can be used the first group result.

Example -
Expression - (.*)
Matching Text - Some Text
Group 1 - Some Text

So using above, below will be your updated fluentd conf.

<match **>
 @type rewrite_tag_filter
 <rule>
  key $["kubernetes"]["aaa"]["bbb"]
  pattern "(.*)"
  tag $1
 </rule>
</match>