I for instance can search for "rabbit" and find something, but find nothing by searching for "rabbi*" or "rabbi%".
Searching (in Postgres on MediaWiki) works on word stemming, so that any search term is broken down to its base, and the base words are compared against an indexed. There is no support for searching arbitrary collections of letters, which seems to be what you want. Doing such a thing would be an extremely time-consuming task on all but the tiniest of wikis, as there is no way to index such arbitrary searches. If this is something you really wanted, however, you could probably write an extension and/or global flag to do the searching by producing SQL similar to:
SELECT * FROM page_content WHERE old_text ~ 'rabbi' WHERE ...