FIX: Avoid matching tag names that contain the current tag name.

The `.\{-}` match for the remainder is too lax; it can pick up intermediate tags where the current tag name is a subset of, e.g. the GSP `<g:else>` and `<g:elseif>` tags.
If there's content after the tag name, it must be separated by whitespace.
Also, as no attributes are allowed on the end tag, the pattern can be restricted to only optional whitespace.