How else would you write that you want to match all strings that don't contain string X? If you were matching at a specific position, you should use a negative lookahead (?!xyz), but I think in some cases you might need the mess above.
Negative lookahead is the only way I can imagine this being possibly useful.
I.E. "Give me a string that's not trump and has a vowel in it".
Given "trunk", that mess above would match all of "trun". Would good is matching a prefix going to do ever?
That part could be useful, but not when the pattern matches a seemingly random number of characters in the process.
[^x] is more predictable. It matches a single character, or not at all.
Well, then by application of your name, it should be useful for any number of characters. They're not random -- they spell out a word you would like not to match.
I'm not sure what you mean with "predictable". Are you referring to readability of the expression? I totally give you that... which kinda was my whole argument to begin with.
My opening statement was that I can't see a practical use for it. I still can't. Maybe one exists. But I don't think we're getting anywhere.