Hi @rjthibod,
Have you looked at this documentation http://docs.splunk.com/Documentation/Splunk/latest/Admin/Transformsconf ?
Specially this one
REGEX and the FORMAT attribute:
* Name-capturing groups in the REGEX are extracted directly to fields.
This means that you do not need to specify the FORMAT attribute for
simple field extraction cases (see the description of FORMAT, below).
* If the REGEX extracts both the field name and its corresponding field
value, you can use the following special capturing groups if you want to
skip specifying the mapping in FORMAT:
_KEY_<string>, _VAL_<string>.
* For example, the following are equivalent:
* Using FORMAT:
* REGEX = ([a-z]+)=([a-z]+)
* FORMAT = $1::$2
* Without using FORMAT
* REGEX = (?<_KEY_1>[a-z]+)=(?<_VAL_1>[a-z]+)
* When using either of the above formats, in a search-time extraction,
the regex will continue to match against the source text, extracting
as many fields as can be identified in the source text.
So are you giving field name in your REGEX ?
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