No regex in my mind :-( I think your string is a kind of tree with nodes, and your leaves are the most internal letters. It's not impossible a struct C-like and isolate every level of nodes and the leaves at the last level.

( a ( b ( c ) ( d ) e ) )

Graphically:

     a
    / \
   b  e
  / \
 c   d

Excuse me for my english