C++ regex - replacing substring in char

Question

I have written the following function to replace substrings in a char. This way involves converting to a std::string then converting back to a const char. Is this the most efficient way or could I do it without this conversion or even a better way?!

    const char* replaceInString(const char* find, const char* str, const char* replace)
    {
        std::string const text(str);
        std::regex const reg(find);
        std::string const newStr = std::regex_replace(text, reg, replace);

        //Convert back to char
        char *newChar = new char[newStr.size() + 1];
        std::copy(newStr.begin(), newStr.end(), newChar);
        newChar[newStr.size()] = '\0'; // terminating 0

        return newChar;
    }


    const char* find = "hello";
    const char* replace = "goodbye";
    const char* oldStr = "hello james";

    const char* newStr = m->replaceInString(find, oldStr, replace);

Answer 1

Assuming that you want a function to be called from a .c file you could use strdup (see code below).

#include <string>
#include <regex>
#include <cstdio>
#include <cstdlib>

char* replaceInString(char const *find, char const *str, char const *replace)
{
  return strdup(std::regex_replace(std::string(str), std::regex(find), replace).c_str());
}

int main()
{
  char *newStr = replaceInString("hello", "hello james", "goodbye");
  printf("newStr = %s\n", newStr);
  free(newStr);

  return 0;
}

Note however that you have to free the returned memory after you're done.

Otherwise, as @jerry-coffin suggested go all the way with std::string (see code below):

#include <string>
#include <regex>
#include <iostream>

std::string replaceInString(std::string const &find, std::string const &str, std::string const &replace)
{
  return std::regex_replace(std::string(str), std::regex(find), replace);
}

int main()
{
  std::string str = replaceInString(std::string("hello"), std::string("hello james"), std::string("goodbye"));
  std::cout << str << std::endl;
  return 0;
}