Assume you have a log file with criticalities associated with them: INFO, ERROR, CRITICAL, etc.
You want all log lines except INFO.
You could use a filter like is explained at:
regex - Regular expression to match a line that doesn't contain a word? - Stack Overflow
^((?!INFO).)*$
Saves you from having to know what to include.
Alternatively what if you need to match a log line with words in it but they could be any order:
Regex to match string containing two names in any order - Stack Overflow
^(?=.*\bERROR\b)(?=.*\bCRITICAL\b).*$
-Garin