Difference in regex_replace Replacement Format Handling Between Boost.Regex and std::regex

There is a difference in how the replacement format string is handled by regex_replace in Boost.Regex and std::regex.

Consider the following example:

```cpp
#include
#include
#include

int main() {
std::string s{"a:b:c"};
std::string rx{":"};
std::string r{"\\$&"};

// Using std::regex
std::string result_std = std::regex_replace(s, std::regex(rx), r);
std::cout << "std::regex result: " << result_std << std::endl; // Outputs "a\:b\:c"

// Using Boost.Regex
std::string result_boost = boost::regex_replace(s, boost::regex(rx), r);
std::cout << "Boost.Regex result: " << result_boost << std::endl; // Outputs "a$&b$&c"

return 0;
}
```

Affects Boost.Regex 1.84. I didn't try other versions.

Tested with GCC 13.2, clang 18.10 , cl 19.39.33523 and results are the same. Not tested with other versions.