> My guess is that it is horribly explosive in N.

I doubt it.

Intuitively I'd say a two phase approach should work.

• Build a tree like with a Trie
• Parse the tree bottom-up and collapse it into a graph by using look-up hashes for duplicated sub-trees

Since a Trie-tree isn't "exploding", this should be safe.

Untested, of course.

UPDATE

Sorry, I should have read the examples in the OP more closely.

I wasn't aware that

• Or-groups were nested a{b,{c,d}}e (not even that it's possible)
• empty alternatives were allowed a{b{,c}}d

Without that my approach above should work.

Now I'm rather pessimistic.

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