I have recently started using the regex feature in the Edit Toolbar's Search and Replace.
I have been unable to get a regex using a lookahead to successfully replace the text. As an example, here - http://en.wikipedia.org/w/index.php?title=User:Sillyfolkboy/Resources/Ath_Sandbox_1&action=edit
Using "\d (?=\w)" successfully finds all the lines starting with a digit and three spaces (followed by a letter). When I click "replace all" I'm told that 123 replacements have been made, but no text is replaced. Clicking find next, then replace all tells me that 123 replacements have been made again. The replace function seems to fail when a lookaround is included in the search.
This may be just a learning issue for myself given the lack of documentation (see https://bugzilla.wikimedia.org/show_bug.cgi?id=23626) but what I'm doing works on http://www.regextester.com/ which ostensibly should be doing the same thing as Wikipedia's search and replace.