Technically it's not a null character -- I think I mis-named it. It's a zero-width assertion, like \b. From the Camel:

Notice that a tab is one character wide, while a word boundary is zero characters wide because it's the spot between two characters. So we call \b a zero-width assertion.

As far as further explanation goes, perhaps someone who is more of an expert at this (which is a large number of people ;) ) can step in...