I've thought about this too and it looked like a tough problem to solve, but it isn't.
Why do people do that?
They usually do it for Ask HN posts and well you can easily discriminate for them. Further if anyone tries to spoof the system by adding a URL there then we can parse it to see if there is any text there according to the stupidity filter. So, essentially it will still work.
Comments
I've thought about this too and it looked like a tough problem to solve, but it isn't.
Why do people do that?
They usually do it for Ask HN posts and well you can easily discriminate for them. Further if anyone tries to spoof the system by adding a URL there then we can parse it to see if there is any text there according to the stupidity filter. So, essentially it will still work.