I was playing around with pcre library, and found out that it provides two matching algorithms:
The NFA algorithm (pcre_exec) doesn't find the longest possible match
The DFA algorithm (pcre_dfa_exec) doesn't support backreferences.
I see it as a drawback that pcre can not simulaneously provide these two features. For example boost regex library is able to do that.
I think this aspect should be taken into account before selecting pcre for integration into U++ library.