It seems that find and replace for a regular expression match doesn't work on English Wikipedia. I've tested this largely on Windows 10 Edge.
Topic on Talk:2017 wikitext editor
Would you please share more information about what exactly you were trying to do? It might be a general problem, but also just a browser-specific problem or a JavaScript limitation.
I ran a regex, I think
with replacement value (?<=\d{3}) (?=BCE?)
; replacement simply didn't work. I exported the text to Sublime Text where it ran fine.
Indeed, that doesn't work for me either. (\d{3}) (BCE?)
→ $1 $2
does, though.
This issue still isn't fixed. Running a regular expression
<nowiki>\|url=.+?worldcat.+? (?=\|)</nowiki>
yields the same problem.
Reported as phab:T336635.