STL и string

#include <iostream>
#include <fstream>
#include <algorithm>
 
using namespace std;
 
int main()
{
ifstream in("text1.txt");
string str,txt,str2;
while(!in.eof())
  {
  getline(in,str);
  txt+=str+'\n';
  }
cout<<"enter search word : "<<endl;
cin>>str;
cout<<"enter cin word : "<<endl;
cin>>str2;
str2=" "+str2+" ";
string::iterator it1=txt.begin();
int i=0;
 
while(it1!=txt.end())
  {
  cout<<*(it1=search(it1,txt.end(),str.begin(),str.end()));
     if(it1!=txt.end())
  txt.insert(it1,str2.begin(),str2.end());
  }
cout<<endl<<txt;
}

  std::string text;
  std::string output;
  std::string w1, w2;
 
  std::cout << "Input w1: ";
  std::cin >> w1;
  std::cout << "Input w2: ";
  std::cin >> w2;
 
  //Заносим исходный текст в text
/*
...
*/
  
  std::string::iterator iterlo = text.begin();
  std::string::iterator iterhi = text.begin();
 
  do {
    iterhi = std::search(iterlo, text.end(), w1.begin(), w1.end());
    std::copy(iterlo, iterhi, std::back_inserter(output));
    if (iterhi != text.end())
      output += w1 + ' ' + w2;
    iterlo = iterhi + w1.size();
    iterhi = iterlo;
  } while(iterhi != text.end());

iterator insert (iterator position, const value_type& val);

#include <iostream>
#include <string>
 
int main() {
    std::string origin = "list string vector list map map list vector string list";
    const std::string search_str = "list";
    const std::string additional_str = "&&";
    
    size_t pos = 0;
    
    while ((pos = origin.find(search_str, pos)) != std::string::npos) {
        pos += search_str.size();
        origin.insert(pos, additional_str);
        pos += additional_str.size();
    }
    
    std::cout << origin << std::endl;
    
    return 0;
}

#include <iostream>
#include <string>
#include <regex>
 
using namespace std;
 
int main() {
    const string origin = "list string vector list map map list vector string list";
    const string search_str = "list";
    const string additional_str = "&&";
    regex e(search_str);
    string result = regex_replace(origin, e, "$0" + additional_str);
    cout << result << endl;
    return 0;
}