/filter excludes matched lines. I am requesting a way to include rather than exclude. I am thinking of something like the / search in the multitail utility, or the -v, --invert-match parameter of grep.

Maybe the inversion is part of the filter definition, or maybe it is an action (toggle) that is applied TO an existing filter.

Example use cases would be to only show posted trades or chat for a certain stock ticker in a trading channel, or only show lines matching my nick (it would be nice if the search could scan the nick AND the line for this, in order to display only my posts and replies (where my nick was quoted)).

(These examples have more of the character of a temporary search than a permanent filter so I allow that /filter may not be the best fit).