Serhiy, I expect this is easy to implement _inside_ the regexp engine.  The complications come from trying to do it outside the engine.  But even there, wrapping the original regexp <re> in

(?:<re>)\Z

is at worst very close.  The only insecurity with that I've thought of concerns the doc's warnings about what can appear before an inline re.VERBOSE flag.  It probably works fine even if <re> does begin with  (?...x....).