Use the s/ sed expression with a user's message, the "sed" flag omits the need for prepending $sed
Examples:
* s/foo/bar - replace foo with bar
* s/foo/bar/2 - replace the 2nd match of foo to bar
* s/foo/bar/someone - replace foo with bar from a message composed by the nick "someone"
* s/foo/bar/g - replace ALL matches of foo to bar