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And lo and behold: "shopt -s extglob" is on the bashrc of the system where this works and that I was ssh'd into when I originally tested this
(No idea why, the particular file grew over the past 15 years…)
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I'm not even entirely sure why it is disabled by default, since it doesn't really break compatibility if you do, it just adds new features which as you observed are otherwise syntax errors.
Managing AUR repos The Right Way -- aurpublish (now a standalone tool)
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I googled and the interwebz says: backward compatibility only (ie. prevent you from bash incompatible bashisms)
bash apparently also has a config option to enable it by default, so there seem to be no concerns at all.
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