Hello, I'm trying to find word boundaries with regex_search and "\b" pattern. I have the following code: string buff = "aaaa*******\\b"; boost::match_flag_type flags = boost::match_default | boost::match_not_null; boost::regex re("\\b"); boost::match_resultsstring::const_iterator what; string::const_iterator start = buff.begin(); string::const_iterator end = buff.end(); if ( regex_search(start, end, what, re, flags)) { string out(start,what[0].second); cout << "match : " << out << endl; } I would expect it to print "aaaa" in out string and to match the second boundary, but in fact it didn't match anything. If I remove the match_not_null flag then it will match the first boundary at the beginning of the string as expected. I don't understand this behavior, am I missing something? Any help would be highly appreciated. Thanks, Roman