The previous implementation of punescape nesting only supported one
layer, because I didn't want to rewrite the pure re.sub approach.
But regex is not a pushdown automaton!
I.e. it's impossible to match both of these correctly:
%{a%}s%{b%}s
%{a%{a%}s%}s
This rewrites punescape and psplit to properly parse the punescape
string, recursing when we encounter "%{" and terminating on "%}s".
As a plus this shows that the punescape grammar is sound. This was a bit
up in the air with the hacky regex globbing.
Christopher Haster
d23edafcfe