Regex to remove trailing slash not working
I think you are getting the twig replace filter (which does not have the regex capability) instead of the Craft replace filter (which does).

So this is a bug in Craft, in that their filter is masked by the twig one.

You could use the trim filter to remove trailing /s:

website|replace({'http://':''})|trim('/')


Or you could use one of Victor's Twig PCRE Filters.