[gwd]

The X asked for a regex, not a command-line parameter. It's possible that the user was actually using POSIX grep, in which case Y ("don't invert the regex, use this command-line parameter to invert what's done with the result of the regex") is the answer they actually need. It's also possible that they're using some other tool, which they know implements POSIX-compatible regexes, but doesn't have an easy way to invert the usage; in which case the user really does need an answer to X (i.e., a way to invert the regex).