https://en.wiktionary.org/w/index.php?search=-happy&title=Special:Search
https://en.wiktionary.org/w/index.php?search=-blablabla&title=Special:Search
All such searches return the same set of random results:
- vīriešu
- 하십시오체
- viņas
- paykuna
- -li
etc.
Exact title matches are found: https://en.wiktionary.org/w/index.php?search=-et&title=Special:Search&fulltext=1 But that is the only useful result you get.