Warn me about too long urls *before* they get posted

For some reason I was able to paste in a tiny url to the text box on the Web form that pushed the message over 140 characters. I double-checked that it showed the whole url, since the "characters available" was at 0. It posted it but truncated the url. After it posted I got a big alert message that said something like "Whoah, your post was too long.

What I *wish* had happened was that I got a warning before the message posted so I could have avoided posting a broken url.