No application crash, the only changes is in std::string stmp = "PORT (127,0,0,1,6,56)\r"; this works with boost::regex regex_ip(".+\\(([0-9]{1,}),([0-9]{1,}),([0-9]{1,}),([0-9]{1,}),([0-9]{1,}),([0-9]{1,})\\).*"); std::string sip = boost::regex_replace(stmp, regex_ip, "$1.$2.$3.$4", out 127.0.0.1 to std::string stmp = "PORT 127,0,0,1,6,56\r"; this not. out PORT (127,0,0,1,6,56) if i change to boost::regex regex_ip(".+\([0-9]{1,}),([0-9]{1,}),([0-9]{1,}),([0-9]{1,}),([0-9]{1,}),([0-9]{1,})\.*"); the out is 7.0.0.1 I must change something in the regex_ip but i cant figure it out. ~~~~~~~~~~~~~~ You're regular expression has literal ( and ) in it - the \\( and \\) parts, so.... not surprisingly these will only match if the text has literal ( and )'s. So either remove those from the expression or make them optional by using \\(? and \\)?. HTH, John.