I guess it's difficult to fix it now because regexes are now merged (piped) first; people would modify instructions to "fix" it now...

Maybe you can use (?<!-)\bt\.co\b?

The regex suggested while functional for a regex is unsuccessful in preventing addition of the link. [Tested by two people on two different wikis with linked being added]. So back to square one.

So we are back to the situation that the spam blacklist is not completely functional for regex to prevent addition of just t.co

^t\.co\b FAIL
(?<!-)\bt\.co\b FAIL

(In reply to billinghurst from comment #2)

The regex suggested while functional for a regex is unsuccessful in
preventing addition of the link. [Tested by two people on two different
wikis with linked being added]. So back to square one.

So we are back to the situation that the spam blacklist is not completely
functional for regex to prevent addition of just t.co

^t\.co\b FAIL
(?<!-)\bt\.co\b FAIL

In my test, [[MediaWiki:Spam-blacklist]]:

#<pre>
google
(?<!-)\bt\.co\b
baidu
#</pre>

Link:

http://t.co/abc

and it says:

The following text is what triggered our spam filter: t.co

... so it works for me?

Yepp, it works. It didn't last time probably because the list was not updated when it was tested. The documentation at mww is wrong.

Actually the instructions at mw:Extension:SpamBlacklist#Blacklist syntax <s>are</s>were wrong, because all blacklisted domains will be php-joined like https?://[a-z0-9.-]*(sbl0_|sbl_1|...|sbl_n).
So if one want's to block an exact domain, the code
(?<=//|\.)t\.co\b
can be used, i.e., "a 't\.co\b' which is preceded either by two slashes or a dot".
We do that already at w:de, w:en and at meta for years now. I agree, it's better to fix the instructions rather than changing the code. I've done that now. -> won't fix?