All rights reserved. Issue D These questions address areas of imprecision in the specifications of REs. A subexpression that can match a null string shall not be duplicated. The latter alternative is consistent with the rule that a null string is longer than no match. By extending xxx with a null string instead of nothing, one gets a longer match.
More null strings make even longer matches.
To avert metaphysical questions about the last element of an infinite sequence, or an element following an infinite sequence, one could forbid null iterations. Compromise positions might forbid null iterations after the first iteration or after the first null iteration. Such special pleading and the resultant implementation complexity is not worth the return. The set of regular languages is unchanged, however. There would still be legal equivalents to these and all other outlawed expressions.