The Regex siteaccess matcher can't really be reversed, a feature required by cross-siteaccess links generation (
André suggested that we just deprecate it.
I (BD) kind of agree, considering that I can't think of a use-case that couldn't be implemented with other matchers. A regex matcher clearly applies to a situation with a non exhaustive list of siteaccesses, like a micro-sites factory. Anything with a non finite list could easily be achieved using Map based matching, along with a statically generated yaml file.
Furthermore, a matcher whose siteaccesses can't be linked to can be an issue if somebody uses the regex matcher without paying attention to the fact that he can't create links.
Jérôme isn't really sure it should be deprecated.