(?-xism:^foo$)
(?-xism:(?-xism:^foo$))
(?-xism:(?-xism:(?-xism:^foo$)))
(?-xism:(?-xism:(?-xism:(?-xism:^foo$))))
(?-xism:(?-xism:(?-xism:(?-xism:(?-xism:^foo$)))))
(?-xism:(?-xism:(?-xism:(?-xism:(?-xism:(?-xism:^foo$))))))
...