Interesting; I've never had to think about how regex interact with Unicode.
I guess what I really meant is that programmers are talking about PCRE in terms of power, not in terms of exact syntax. In particular, they have recursive patterns, which are sufficient to pull them up to context-free grammars.
I wish that were the case. http://www.regular-expressions.info/refunicode.html
(They used to have a much more useful and concise comparison table but I can't for the life of me find it.)