The topic Check if a date string is in ISO and UTC format is focused on full ISO8601 strings including the time part, so as it has been clearly declared, the regular expression from this solution will fail if the string will include only the date part like 2018-10-18. This question is about ISO8601 strings only with date part.
AFAIK the date part in ISO8601 string including 10 characters, but new Date("1970-1-01") (9 characters) will work. So I suppose, the first question will be: are first 10 characters reserved for the date part in ISO8601 string? If so the /^d{4}-d{2}-d{2}$ should works. If no, I afraid , the regular expression could be too complicated. Maybe new Date(targetISO_String).toString() === "Invalid Date will be enough?