private void subFlag() {
        int ch = peek();
        for (;;) {
            switch (ch) {
            case 'i':
                flags &= ~CASE_INSENSITIVE;
                break;
            case 'm':
                flags &= ~MULTILINE;
                break;
            case 's':
                flags &= ~DOTALL;
                break;
            case 'd':
                flags &= ~UNIX_LINES;
                break;
            case 'u':
                flags &= ~UNICODE_CASE;
                break;
            case 'c':
                flags &= ~CANON_EQ;
                break;
            case 'x':
                flags &= ~COMMENTS;
                break;
            case 'U':
                flags &= ~(UNICODE_CHARACTER_CLASS | UNICODE_CASE);
            default:
                return;
            }
            ch = next();
        }
    }