I know this is a very old topic, but I had the exact same issue and found that none of the text replacement plugins worked for me. I don't want to have to go into configuration and tweak settings every time I need to do a search/replace.
I ended up thinking outside the dokuwiki box. I found a chrome extension that does search/replace on text boxes on web pages. it is very fast and easy to use.
https://chrome.google.com/webstore/detail/search-and-replace/bldchfkhmnkoimaciljpilanilmbnofo?hl=en-GB
The only thing that would make it better is if it gave some sort of verification of how many instances of the search string were found/replaced.
hope this helps someone else too :)
Taylor