[kh99]

Yeah, conditionals aren't allowed, and if they were they probably wouldn't work like you'd expect because of post caching. The message text (and bbcode) is processed and saved so it doesn't have to be done every time someone views a post. But what you can do is fake it by writing a plugin and doing your own processing (like a str_replace()) on the message text before it's displayed (and that's obviously going to add some overhead). Also, you'd be on your own for providing a button in the editor.

What do you mean by "as to what the user inputs"? What input did you want to check exactly?