Excuse me while I completely ignoring your interesting question about hash inversions. But, have you tried XML::Twig, it is usually a good choice for dealing with large XML trees.

Returning to your question... usually you need a push down stack to handle recursive parsing like you are describing. When you see an opening tag, push it. When you see a closing tag pop it. In between walk up or down the stack's array to see where you are at present.

Phil

In reply to Re: Nesting below emptiness and/or inverting a hash. by philcrow
in thread Nesting below emptiness and/or inverting a hash. by Moron

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