Steps to replicate the issue (include links if applicable):

• Open a blank sandbox page.
• Fill it with "123 BCE".
• Push Ctrl+F, find: "(?<=\d{3}) (?=BCE?)", replace: "foo", regex modifier on.
• Click "Replace".

What happens?:

No replacement is done, although the match is highlighted and counted in "1 of 1".

What should have happened instead?:

The replacement is made.

Other information (browser name/version, screenshots, etc.):

Concerns the visual editor as well as 2017 wikitext editor.
Reported in https://www.mediawiki.org/wiki/Topic:Xgi8ideuh8uiyo1p.