to make it so that only "throw a" as in "throw a TypeError exception" counts, so that a reference to a "throw completion" isn't interpreted as throwing,
to make "Return Completion Record { ... }" count as returning a completion record, and
to distinguish between a reference to a "return completion" and a step which says to "Return Completion (...)".
Regex change is necessary for https://github.com/tc39/ecma262/pull/3163 to land.
The change to the regexp is