So, what's actually an syntactically impossible path? Can you say much more than what a tr/// check says? (Untainting can be done with a fake-regexp like /(.*)/, you don't need something complicated for that.) What can you really exclude? Not much Apart from /\.\.\.+/, or?

Bit still I can't see the sense of that, why do you want to check whether a filename is syntactically possible. It doesn't give you any hints about the actual possibility of creating or finding such a file...

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