[re.synopt]/1 says "A valid value of type syntax_option_type shall have exactly one of the elements ECMAScript, basic, extended, awk, grep, egrep, set."

This "exactly one" wording technically forbids passing icase by itself! Users should not be required to pass regex::ECMAScript | regex::icase. (Note that the cost of an additional check for no grammar being explicitly requested is completely irrelevant, as regex construction is so much more expensive.)