I'm pretty decent with regex, but I don't know or use it's full syntax. For example, the (<=stuff) idiom is new to me. I've never used it and I don't know exactly how it works.
It sounds like you've got it under control. If you need more advice, let me know and I'll try to help.
Edit: I couldn't help myself and had to try it myself. Here's my take:
regex([Description],/#.+ at ([^\s]+)#/,-1,0)[R1]
The "trick" here is the stuff I'm matching on inside the () isn't just a dot character (which can be anything). Instead I'm matching on [^\s]. Which means "match any character except space type characters". So it stops the match when it hits the first space, tab, or other whitespace type character.
Seems a little more readable and simple to me... but then again, I wrote it, so it should seem easier to understand for ME.
Brian.