If the behavior is obviously wrong (like in issue25054), we can fix it without warnings, and even backport the fix to older versions, because we do not expect that anybody depends on such weird behavior. If we are going to change the behavior, but expect that users can depend on the current behavior, we emit a FutureWarning first (and we did it for other changes in re). But this issue is the hard one. Before 3.7 we did not know that it is related to issue25054. We were not going to change this behavior (at least not in near future). But when a fix for issue25054 was written we did see that it is the same issue. We did not want to keep a bug in issue25054 few versions more, so we changed the behavior in this issue without warnings. It was an exceptional case.

This change was documented, in the module documentation, and in "What's New in Python 3.7" (section "Porting to Python 3.7"). If this is not enough we will be happy to get help to make it better.