hover question, checking "active" status

hover question, checking "active" status

I think this is just a dumb question, I'm missing some basic logic here about jquery "grammar".

I'm trying to do an image swap on hover that checks to be sure the thumbnail is not the active thumbnail. So mouseover, it swaps. Mouseout, swaps back, unless the thumb has been clicked.

here's a snippet of the code that doesn't work:
thumb.hover(
               function() { this.src = this.src.replace('b.','.'); },
               function() { _li.not('.active').children('src').replace('.','b.'); } // this does not work
               // this works but doesn't keep the active thumb "active":
               //function() { this.src = this.src.replace('.','b.'); }


how do I check that _li is not ".active" and replace the src if it isn't?

thanks!!