Depending on the context, <something> | "" (or "" | <something>) may be a clearer way to spell out Opt(...); I believe this almost works except for the fact that Literal("") currently raises an error asking to use Empty() instead. Could this restriction be lifted?

I see various ways to lift the restruction, depending on what you are exactly willing to accept. One would be to just directly allow Literal("") (if you worry about the loss in perf, you could define Literal.__new__ to return Empty() in that case, although that's a bit too magical in my taste). Or you could change ParseElementEnhance.__init__ at

to special-case expr == "" there (or rather len(expr) == 0 to support bytes too) and return Empty(). Finally, you could also change ParserElement.__or__/__ror__ to detect that case and construct an Optional(...) as needed (this may be the simplest from an explanation point of view: "| constructs a MatchFirst, except that | "" constructs an Optional").

I can make a PR if you agree with any of these solutions.