Telegraf filter out bad data

Question

I have some devices that have, let's say poor SNMP implementation and sometimes it returns junk data instead of the proper data. I am looking for a way to ignore the junk data before it goes into my InfluxDB instance. It's really a matter of a simple regex, only pass data if it matches ^(\D.*), so things like "10025" don't get passed in, only things like "Austin" or "Chicago". Since this is a tag in the measurement, I don't want crap data that I constantly have to manage going into the series at all. It's probably something simple I'm missing in the docs somewhere.