Alternative title: the true danger of conflating regular expressions and backtracking pattern matchers. I wonder if it would be possible to reintroduce that distinction.
Alternative title: you can use regex derivatives (PCRE, Ruby ::Regexp, Boost.Regex, ...) to match non-regular syntax but you shouldn't.
The advice still holds. In the case of HTML you should reach for an XML parser.
(\w+) \d+ \1
This is particularly confusing because you mean "derivative" in the sense of "etymologically/conceptually related", but there is actually a (formal) notion of the derivative of a regular expression.