Simple. The exclamation points are the delimiters for the expression (to avoid having to backslash the forward slash). The expression says: remove everything up to the last '/' from the expression including the '/'.
Remember that the '*' modifier is greedy and will suck up all the '/'s in the expression. To get rid of everything up to the first slash you can use the *? modifier, which does a non-greedy match

-pete
"Pain heals. Chicks dig scars. Glory lasts forever."