[Atherton25]

That's a user ID, not a storyID. Note the /u/ not /s/. It can easily be adjusted to pull one (or both) but you really don't want to extract a userID into a storyID column.

But, for your specific question/request:
(.*?) href="http(s?)://www.fanfiction.net/u/(.*?)/(?s).*
-or-
([\s\S]*) href="http(s?)://www.fanfiction.net/u/(.*?)/([\s\S]*)
will isolate and pull out the ID you're looking for.

Change /u/ to /s/ if you want only story ID's. The matching item is: \3 for the "replace with" box.

It's possible to do:
(.*?) href="http(s?)://www.fanfiction.net/(u|s)/(.*?)/(?s).*
-or-
([\s\S]*) href="http(s?)://www.fanfiction.net/(u|s)/(.*?)/([\s\S]*)
and use \4 - but keep in mind by doing this you're not distinguishing between what type of ID it is, so I would avoid that unless you have a specific reason for it.

NOTE: edited above string from initial post, to allow for additional lines of text Not sure what the before/after text looks like, but that should take care of anything else.

Anything newer seems to be HTTPS on ff.net, but I have a lot of older stuff, so I always use the http(s?) function to ensure that anything that's http:// is captured as well.