An earlier iteration of a duplicated subpattern is to the left of a later iteration. Neither side of an alternation is to the left of the other. Length ties in an alternaion are broken in favor of the left side. The proposed solution adopts first- finished. In many cases, including the present example, first- finished matching can be done with less backtracking.
Historical practice favors first-finished.
Thus first-begun matching entails a contradiction. The standard does not require the successful matching of a null string after a successful match on a non null string for duplication symbols. However, the standard does not clearly say that you are not allowed to match null strings after a successful match. Therefore the standard in ambiguous. The definition of a back reference expression in subclause 2.8.3.3 does not specify which match of a subexpression followed by a duplication symbol is to be returned.