generating {|} in math mode

[Micha Feigin]

how do I generate a group definition such as
{ x | x > 0 }
with variable height braces? I can create the outside braces using left\{ and 
right\} (or matching braces from the braces menu) but the central one doesn't 
fit it, and if I add a | with a matching empty left or right element the sizes 
don't match