Steps to Reproduce:
Submit the query `insource:/\//` or `intitle:/\//` in CirrusSearch.
Practical example link with narrowed search domain: [[https://en.wikipedia.org/w/index.php?search=%3A+slash+insource%3A%2F%5C%2F%2F&title=Special:Search | Search results for “: slash insource:/\//”]] (English Wikipedia).
Error message: `An error has occurred while searching: Regular expression syntax error at unknown: unknown`. Apparently there is some kind of parsing error.
Should search for pages containing `/` or titles in the given namespaces containing `/`. The query also fails if more valid regular expression characters are added before `\/`: `insource:/word\//` or `intitle:/word\//`.
Search succeeds when `\/` is not the last thing in the regex: `insource:/\/./` or `intitle:/\/./`.
Again, example link for English Wikipedia: [[https://en.wikipedia.org/w/index.php?search=%3A+slash+insource%3A%2F%5C%2F.%2F&title=Special:Search | Search results for “: slash insource:/\/./”]]
This is probably a separate issue, but `insource://` and `intitle://`, and `insource:///` and `insource:////`, etc., have an odd error: `An error has occurred while searching: We could not complete your search due to a temporary problem. Please try again later.` I would've expected something like "You can't search for an empty regular expression" and "Invalid syntax: `/` found after `insource://` syntax, expected space character or end of query".