I've tried all sorts of things but I think i'm getting too complicated and even managed to get chrome to hang with my selector. I'm sure theres a simple way to do this
Select classa
but only when theres no classb
and ignore the last instance of it
<div class="container">
<div class="classa classb"></div> <!-- Dont Select -->
<div class="classa"></div> <!-- Select -->
<div class="classa"></div> <!-- Dont select last instance -->
</div>