Perl 6 extends the regex system to be a full fledged recursive descent parser, which lets you do this kind of stuff. But as is pointed out in a sibling comment,

Perl 6 extends the regex system to be a full fledged recursive descent parser, which lets you do this kind of stuff. But as is pointed out in a sibling comment, a purely regular regex engine won't really do it since it's non-regular language. Of course, some regex engines do let you do this (notably Perl 5), but it requires code of the "just because you can doesn't mean you should" variety.