Are you sure you're asking in the right thread? OCaml has a binding to the PCRE library: https://mmottl.github.io/pcre-ocaml/

...the question seems moot to me.

If you need a regex that complicated it’s probably better to be written as a parser. Not only will it be way more performant, it will be much more maintainable.

I agree that if you're designing a system from scratch this makes sense. Ocassionally you need to extend the syntax of an existing system that is written with regular expressions and would require a negative lookahead. Some sub-parser with negative lookahead functionality would be possible, but a regex would be more convenient in a pinch.