Replace substring with another substring C++

How could I replace a substring in a string with another substring in C++, what functions could I use?

eg: string test = "abc def abc def";
test.replace("abc", "hij").replace("def", "klm"); //replace occurrence of abc and def with other substring

In [tag:c++11], you can use std::regex_replace:

#include <string>
#include <regex>

std::string test = "abc def abc def";
test = std::regex_replace(test, std::regex("def"), "klm"); // replace 'def' -> 'klm'
// test = "abc klm abc klm"

There is no one built-in function in C++ to do this. If you'd like to replace all instances of one substring with another, you can do so by intermixing calls to string::find and string::replace. For example:

size_t index = 0;
while (true) {
     /* Locate the substring to replace. */
     index = str.find("abc", index);
     if (index == std::string::npos) break;

     /* Make the replacement. */
     str.replace(index, 3, "def");

     /* Advance index forward so the next iteration doesn't pick it up as well. */
     index += 3;
}

In the last line of this code, I've incremented index by the length of the string that's been inserted into the string. In this particular example - replacing "abc" with "def" - this is not actually necessary. However, in a more general setting, it is important to skip over the string that's just been replaced. For example, if you want to replace "abc" with "abcabc", without skipping over the newly-replaced string segment, this code would continuously replace parts of the newly-replaced strings until memory was exhausted. Independently, it might be slightly faster to skip past those new characters anyway, since doing so saves some time and effort by the string::find function.

Hope this helps!

Boost String Algorithms Library way:

#include <boost/algorithm/string/replace.hpp>

{ // 1. 
  string test = "abc def abc def";
  boost::replace_all(test, "abc", "hij");
  boost::replace_all(test, "def", "klm");
}


{ // 2.
  string test = boost::replace_all_copy
  (  boost::replace_all_copy<string>("abc def abc def", "abc", "hij")
  ,  "def"
  ,  "klm"
  );
}

I think all solutions will fail if the length of the replacing string is different from the length of the string to be replaced. (search for "abc" and replace by "xxxxxx") A general approach might be:

void replaceAll( string &s, const string &search, const string &replace ) {
    for( size_t pos = 0; ; pos += replace.length() ) {
        // Locate the substring to replace
	    pos = s.find( search, pos );
	    if( pos == string::npos ) break;
	    // Replace by erasing and inserting
	    s.erase( pos, search.length() );
	    s.insert( pos, replace );
    }
}

str.replace(str.find(str2),str2.length(),str3);

Where

• str is the base string
• str2 is the sub string to find
• str3 is the replacement substring

Replacing substrings should not be that hard.

std::string ReplaceString(std::string subject, const std::string& search,
                          const std::string& replace) {
    size_t pos = 0;
    while((pos = subject.find(search, pos)) != std::string::npos) {
         subject.replace(pos, search.length(), replace);
         pos += replace.length();
    }
    return subject;
}

If you need performance, here is an optimized function that modifies the input string, it does not create a copy of the string:

void ReplaceStringInPlace(std::string& subject, const std::string& search,
                          const std::string& replace) {
    size_t pos = 0;
    while((pos = subject.find(search, pos)) != std::string::npos) {
         subject.replace(pos, search.length(), replace);
         pos += replace.length();
    }
}

Tests:

std::string input = "abc abc def";
std::cout << "Input string: " << input << std::endl;

std::cout << "ReplaceString() return value: " 
          << ReplaceString(input, "bc", "!!") << std::endl;
std::cout << "ReplaceString() input string not changed: " 
          << input << std::endl;

ReplaceStringInPlace(input, "bc", "??");
std::cout << "ReplaceStringInPlace() input string modified: " 
          << input << std::endl;

Output:

Input string: abc abc def
ReplaceString() return value: a!! a!! def
ReplaceString() input string not modified: abc abc def
ReplaceStringInPlace() input string modified: a?? a?? def

using std::string;

string string_replace( string src, string const& target, string const& repl)
{
    // handle error situations/trivial cases
    
    if (target.length() == 0) {
        // searching for a match to the empty string will result in 
        //  an infinite loop
        //  it might make sense to throw an exception for this case
        return src;
    }
    
    if (src.length() == 0) {
        return src;  // nothing to match against
    }
    
    size_t idx = 0;
    
    for (;;) {
        idx = src.find( target, idx);
        if (idx == string::npos)  break;
        
        src.replace( idx, target.length(), repl);
        idx += repl.length();
    }
    
    return src;
}

Since it's not a member of the string class, it doesn't allow quite as nice a syntax as in your example, but the following will do the equivalent:

test = string_replace( string_replace( test, "abc", "hij"), "def", "klm")

If you are sure that the required substring is present in the string, then this will replace the first occurence of "abc" to "hij"

test.replace( test.find("abc"), 3, "hij");

It will crash if you dont have "abc" in test, so use it with care.

Generalizing on rotmax's answer, here is a full solution to search & replace all instances in a string. If both substrings are of different size, the substring is replaced using string::erase and string::insert., otherwise the faster string::replace is used.

void FindReplace(string& line, string& oldString, string& newString) {
  const size_t oldSize = oldString.length();

  // do nothing if line is shorter than the string to find
  if( oldSize > line.length() ) return;

  const size_t newSize = newString.length();
  for( size_t pos = 0; ; pos += newSize ) {
    // Locate the substring to replace
    pos = line.find( oldString, pos );
    if( pos == string::npos ) return;
    if( oldSize == newSize ) {
      // if they're same size, use std::string::replace
      line.replace( pos, oldSize, newString );
    } else {
      // if not same size, replace by erasing and inserting
      line.erase( pos, oldSize );
      line.insert( pos, newString );
    }
  }
}

std::string replace(std::string str, std::string substr1, std::string substr2)
{
    for (size_t index = str.find(substr1, 0); index != std::string::npos && substr1.length(); index = str.find(substr1, index + substr2.length() ) )
        str.replace(index, substr1.length(), substr2);
    return str;
}

Short solution where you don't need any extra Libraries.

Here is a solution I wrote using the builder tactic:

#include <string>
#include <sstream>

using std::string;
using std::stringstream;

string stringReplace (const string& source,
				      const string& toReplace,
				      const string& replaceWith)
{
  size_t pos = 0;
  size_t cursor = 0;
  int repLen = toReplace.length();
  stringstream builder;

  do
  {
	pos = source.find(toReplace, cursor);

	if (string::npos != pos)
	{
		//copy up to the match, then append the replacement
		builder << source.substr(cursor, pos - cursor);
		builder << replaceWith;

		// skip past the match 
		cursor = pos + repLen;
	}
  } 
  while (string::npos != pos);

  //copy the remainder
  builder << source.substr(cursor);

  return (builder.str());
}

Tests:

void addTestResult (const string&& testId, bool pass)
{
  ...
}

void testStringReplace()
{
	string source = "123456789012345678901234567890";
	string toReplace = "567";
	string replaceWith = "abcd";
	string result = stringReplace (source, toReplace, replaceWith);
	string expected = "1234abcd8901234abcd8901234abcd890";

	bool pass = (0 == result.compare(expected));
	addTestResult("567", pass);


	source = "123456789012345678901234567890";
	toReplace = "123";
	replaceWith = "-";
	result = stringReplace(source, toReplace, replaceWith);
	expected = "-4567890-4567890-4567890";

	pass = (0 == result.compare(expected));
	addTestResult("start", pass);


	source = "123456789012345678901234567890";
	toReplace = "0";
	replaceWith = "";
	result = stringReplace(source, toReplace, replaceWith);
	expected = "123456789123456789123456789"; 

	pass = (0 == result.compare(expected));
	addTestResult("end", pass);


	source = "123123456789012345678901234567890";
	toReplace = "123";
	replaceWith = "-";
	result = stringReplace(source, toReplace, replaceWith);
	expected = "--4567890-4567890-4567890";

	pass = (0 == result.compare(expected));
	addTestResult("concat", pass);


	source = "1232323323123456789012345678901234567890";
	toReplace = "323";
	replaceWith = "-";
	result = stringReplace(source, toReplace, replaceWith);
	expected = "12-23-123456789012345678901234567890";

	pass = (0 == result.compare(expected));
	addTestResult("interleaved", pass);



	source = "1232323323123456789012345678901234567890";
	toReplace = "===";
	replaceWith = "-";
	result = utils_stringReplace(source, toReplace, replaceWith);
	expected = source;

	pass = (0 == result.compare(expected));
	addTestResult("no match", pass);

}

    string & replace(string & subj, string old, string neu)
    {
        size_t uiui = subj.find(old);
        if (uiui != string::npos)
        {
           subj.erase(uiui, old.size());
           subj.insert(uiui, neu);
        }
        return subj;
    }

I think this fits your requirement with few code!

the impoved version by @Czarek Tomczak.
allow both std::string and std::wstring.

template <typename charType>
void ReplaceSubstring(std::basic_string<charType>& subject,
	const std::basic_string<charType>& search,
	const std::basic_string<charType>& replace)
{
	if (search.empty()) { return; }
	typename std::basic_string<charType>::size_type pos = 0;
	while((pos = subject.find(search, pos)) != std::basic_string<charType>::npos) {
		 subject.replace(pos, search.length(), replace);
		 pos += replace.length();
	}
}

std::string replace(const std::string & in
                  , const std::string & from
                  , const std::string & to){
  if(from.size() == 0 ) return in;
  std::string out = "";
  std::string tmp = "";
  for(int i = 0, ii = -1; i < in.size(); ++i) {
    // change ii
    if     ( ii <  0 &&  from[0] == in[i] )  {
      ii  = 0;
      tmp = from[0]; 
    } else if( ii >= 0 && ii < from.size()-1 )  {
      ii ++ ;
      tmp = tmp + in[i];
      if(from[ii] == in[i]) {
      } else {
        out = out + tmp;
        tmp = "";
        ii = -1;
      }
    } else {
      out = out + in[i];
    }
    if( tmp == from ) {
      out = out + to;
      tmp = "";
      ii = -1;
    }
  }
  return out;
};

Here is a solution using recursion that replaces all occurrences of a substring with another substring. This works no matter the size of the strings.

std::string ReplaceString(const std::string source_string, const std::string old_substring, const std::string new_substring)
{
    // Can't replace nothing.
    if (old_substring.empty())
        return source_string;
        
    // Find the first occurrence of the substring we want to replace.
    size_t substring_position = source_string.find(old_substring);

    // If not found, there is nothing to replace.
    if (substring_position == std::string::npos)
        return source_string;

    // Return the part of the source string until the first occurance of the old substring + the new replacement substring + the result of the same function on the remainder.
    return source_string.substr(0,substring_position) + new_substring + ReplaceString(source_string.substr(substring_position + old_substring.length(),source_string.length() - (substring_position + old_substring.length())), old_substring, new_substring);
}

Usage example:

std::string my_cpp_string = "This string is unmodified. You heard me right, it's unmodified.";
std::cout << "The original C++ string is:\n" << my_cpp_string << std::endl;
my_cpp_string = ReplaceString(my_cpp_string, "unmodified", "modified");
std::cout << "The final C++ string is:\n" << my_cpp_string << std::endl;

std::string replace(std::string str, const std::string& sub1, const std::string& sub2)
{
    if (sub1.empty())
        return str;

    std::size_t pos;
    while ((pos = str.find(sub1)) != std::string::npos)
        str.replace(pos, sub1.size(), sub2);

    return str;
}

#include <string>

First:

void replace_first(std::string& text, const std::string& from,
   const std::string& to)
{
    const auto at = text.find(from, 0);

    if (at != std::string::npos)
        text.replace(at, from.length(), to);
}

All:

void replace_all(std::string& text, const std::string& from,
   const std::string& to)
{
    for (auto at = text.find(from, 0); at != std::string::npos;
        at = text.find(from, at + to.length()))
    {
        text.replace(at, from.length(), to);
    }
}

Count:

size_t replace_count(std::string& text,
   const std::string& from, const std::string& to)
{
    size_t count = 0;

    for (auto at = text.find(from, 0); at != std::string::npos;
        at = text.find(from, at + to.length()))
    {
        ++count;
        text.replace(at, from.length(), to);
    }

    return count;
}

Copy:

std::string replace_all_copy(const std::string& text,
   const std::string& from, const std::string& to)
{
    auto copy = text;
    replace_all(copy, from, to);
    return copy;
}