STREAM_MATCHED for regexp replace ?

mmm... I'm sorry I think and I'm sure I was not clear in my previous post :-) Thank you for your answer aaron, but if I want to re-use this $host variable into the stream replacement ? As you said, I could not use the $host within braces in the stream::expression, so could I do the replacement within the stream_matched event to replace the '/images/blablabla by 'http://media.$host.com/images/blablabla ? Do you have a sample to do that ? Stream::replace ? Thanks