I thought that for_each() goes while (it != end). How does for_each() 'know' what end is here? It doesn't seem to be initialized to anything. Thanks much, Graham class regex_callback { int m_sum; public: regex_callback() : m_sum(0) {} template <typename T> void operator()(const T& what) { m_sum += boost::lexical_cast<int>(what[1].str().c_str()); } int sum() const { return m_sum; } }; int main() { boost::regex("(\\d+),?"); string s = "1,1,2,3,5,8,13,21"; boost::sregex_iterator it(s.begin(), s.end(), reg); // Is end initialized to anything here? boost::sregex_iterator end; regex_callback c; // How is end valid here? int sum = for_each(it, end, c).sum(); return 0; }