Recently I came across to the following page http://www.and.org/vstr/comparison
where James Antill take some time to put together a quite good comprehensive list of C/C++ (mainly) libraries working on strings.

In particular one of those get my attention: bString or "Best String Library"
More info can be found at http://bstring.sourceforge.net/

I was interested mainly because this library already implements some of the BASIC functions working on strings