c++读取与修改文件中的特定数据

Question

            下划线部分的内容修改为我要修改的内容，修改的数据长度不确定，
            且保留其他数据不变

Answer 1

#include<iostream>
#include<fstream>
#include<string>
#include<regex>
using namespace std;
int main() {
    string str;
    int count = 0;
    ifstream in("in.txt");
    getline(in, str);       //读取第一行文本
    in.close();
    regex pat("(-\\w{1,} )([\\d\\w]{1,}) ");  //匹配模式
    regex ip("(([01]{0,1}\\d{0,1}\\d|2[0-4]\\d|25[0-5])\\.){3}[01]{0,1}\\d{0,1}\\d|2[0-4]\\d|25[0-5]"); //存储提取到的数据
    smatch matches;                                     //存储匹配到的字符串
    string replacedate[3] = { "00","11","22" };         //假设依此替换成这些数据
    string result;
    while (regex_search(str, matches, pat)) {
        result += matches.prefix();
        result += string(matches[1]) + replacedate[count++] + " ";    //替换数据
        str = matches.suffix();                     //修改剩余的字符串
    }
    result += str;
    result= regex_replace(str, ip, "255.255.255.255");        //替换ip
    ofstream out("in.txt");
    out << result << endl;
    return 0;
}