#!/bin/bash
s='2020-11-17 15:34:04.208 -0500 Information 94  XYZ-ASDF-FMP123  Client "%USERNAME% (QWER-L1212-W6) [11.22.333.44]" opening database "databasename" as "username".'
grep -o '([^()]*) \[[^][]*]' <<< "$s" | sort -u