Thanks for the swift response.
This is what I use as a testbed:
Every regexp you mentioned, as well as the one I devised myself works fine; i.e. the regexp tester reports that the given string matches the regexp.
Now, neither of them is effective when used with SpamBuster. I made this regexp the first one in SpamBuster's list - so I am certain this is not related to the order in which the rules are written in the config.
Maybe it can be understood by adding some debug statements to the plugin's code and watching the flow of its execution?
Thanks for suggesting Akismet, but my optimism faded away in just a few seconds - I saw that it is marked as "installed", therefore it means that some time ago I tested it and was not satisfied with the results and that my conclusion was that "SpamBuster is better" (-: