Extracting the date from a filename without modifying the datetime.xml

Hi, I'm trying to extract the date from a filename without having to configure the config.file, i want to be able to achieve this through the search panel. This is my file name: name_name_20171130.txt This is the regular expression I've found that is the closest to helping me achieve what I need: source="name_name_20171130.txt" host="xxx" | rex field= source="(?[\d/]+)\s\w+" |table _raw ITEM, SOH_DIFF, field Alternatively, this expression: (?\d{8}) Please Help!