Processor.regex.fields --> pattern & replacement --> default value if not match needed

I have a working telegraf --> processor.regex.fields config.
So, I search for a regex pattern and y apply a replacement:
key = “message”
pattern = ‘(.*?)(&submode=)([^\s\&]+)([^]+)
replacement = “${3}”
result_key = “submode”

Everythink works great but… sometimes no “submode=” chain is present in message and I need a “default” value to be inserted in the influxdb…

Is there any way to apply a default value when there is no match?

Many Thanks in advance!!