Tested in Chrome and Firefox. It does not occur with negative lookaheads/lookbehinds.

Steps to Reproduce:

1. Edit any wiki page in any of these two editors.
2. Press Ctrl+F to open Find & Replace.
3. Enable regex by clicking the "(.*)" button.
4. Type a query followed by a positive lookahead or lookbehind, i.e. "test(?=ing)" or "(?<=test)ing". Make sure it has matches on the page.
5. Click "Replace".

Actual Results:

The matched text is not replaced, and the find & replace tool skips to the next match.

Expected Results:

The matched text is replaced.