LWG2503 added multiline to std::regex_constants (bf6a98d), but did not add it to std::basic_regex, which contains clones of all other syntax options ([re.regex.const] with synopsis in [re.regex]/3)

Does it still qualify as editorial? The text in [re.regex.const] says "The static constant members are provided as synonyms for the constants declared in namespace regex_constants.", but pedantically speaking it doesn't say "all constants defined" and so is not a contradiction.