A better approach would be to use iterators.
// Begin searching 32 characters into altDow regex_search( altDow.begin() + 32, altDow.end(), what, rex )
Note #1: the match results will now contain iterators into altDow, but position offsets will be relative to the start of the search, not the start of altDow.
Is the following getting closer to "best practice" to use xpressive-static for the kinds of specialized searches I'm attempting. When I'm further along on the learning curve, I'll try to prepare some "apples and apples" timing comparisons between boost::regex, xpressive, spirit, a hand-tuned state-machine searcher, and an automatic FSM generator utility. I think I'm conforming to the guidelines on this page: X:\DevTools\Boost\libs\xpressive\doc\html\boost_xpressive\user_s_guide\tips_n_tricks.html except I don't understand how to specify syntax_option_type::optimize (does it apply to xpressive-static, or only xpressive-dynamic)? boost::xpressive::regex_constants::syntax_option_type flags = boost::xpressive::regex_constants::optimize; sregex rexStatic = (s1 = (as_xpr("Sunday") | "Sun")) | (s2 = (as_xpr("Monday") | "Mon")) | (s3 = (as_xpr("Tuesday") | "Tue")) | (s4 = (as_xpr("Wednesday") | "Wed")) | (s5 = (as_xpr("Thursday") | "Thu")) | (s6 = (as_xpr("Friday") | "Fri")) | (s7 = (as_xpr("Saturday") | "Sat")) ; //??? rexStatic.compile(rexStatic, flags); smatch what; std::string testStr = "Alternate days of the week are Tue and Thursday and Sat and Monday. " "And then Monday and Wed and Friday and Sun. "; std::string::const_iterator start = testStr.begin(); std::string::const_iterator finish = testStr.end(); int foundCount = 0; while (regex_search(start, finish, what, rexStatic)) { foundCount++; size_t limit = what.size(); size_t matchIndex = 1; while (matchIndex <= limit) { if (what[matchIndex].matched == true) { break; } ++matchIndex; } #ifdef _DEBUG cout << "FoundCount: " << foundCount << " Index: " << static_cast<int>(matchIndex) << " what[0]: " << what[0] << endl; #endif start = what[0].second; }