The proof for the (x?|y) case follows immediately from regex union being commutative.

Regexp union is not commutative when one of the alternates is a leading substring of another: then order becomes important - (E|x) will always match E in preference to x.

It is the presence of the outer anchors in the original pattern that disambiguates and thus makes it commutative.

Hugo

In reply to Re^2: Why machine-generated solutions will never cease to amaze me by hv
in thread Why machine-generated solutions will never cease to amaze me by grinder

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