C++ - returns copy of passed `src` string with `sep` separators inserted every `group_len` characters counting from left-to-rigth or right-to-left, depending on the `right_to_left` bool param. May be used for example to insert 'thousand separators' into string representations of large numbers. This version has plenty of debugging messages.

string_insert_separators.cpp

#include <format>
#include <iostream>
using namespace std;
/**
 * @brief inserts into the `src` string, every `group_len` characters, the `sep` string; depending on the
 * `right_to_left` parameter starts counting characters from left (when =false) or from right (when =true)
 * It does NOT change the original string passed as an argument
 * @param src source string the separators are to be inserted into
 * @param sep separators to be inserted into the `src` string
 * @param group_len the `sep` string will be inserted into `src` string every `group_len` characters
 * @param right_to_left when =false starts counting characters from left, when =true starts counting from right
 * @return modified copy of `src` string with `sep` separators inserted every `group_len` chars counting l2r or r2l
 */
string insertSeparators(
    const string &src,
    const string &sep,
    const unsigned int group_len,
    const bool right_to_left = false
) {
    cout << "\tInside Params: src: " << src << " sep: " << sep << " group len: " << group_len <<
            " from right: " << right_to_left << endl;
    if (src.empty() || sep.empty() || group_len == 0 || src.length() <= group_len) { return src; }

    cout << "\tInside: " << &src << " " << src << " - src passed seen inside" << endl;
    const auto len_remainder = src.length() % group_len;
    auto position = src.length() -
                    ((right_to_left || len_remainder == 0) ? group_len : len_remainder);
    std::string s = src;
    cout << "\tInside: " << &s << " " << s << " - s declared inside" << endl;
    for (; position > group_len; position -= group_len) {
        s.insert(position, sep);
        cout << "\tInside: " << &s << " " << s << " - s after sep insertion" << endl;
    }
    s.insert(position, sep);
    cout << "\tInside: " << &s << " " << s << " - s after sep insertion" << endl;
    return s;
} // string insertSeparators( … )

int main() {
    const std::string s = "Hello World!";
    cout << endl << "String and their addresses:" << endl << endl;
    cout << "OUTSIDE: " << &s << ", " << s << ", len: "<< s.length()<< ", capacity: "<< s.capacity() <<" - original string s passed as arg to function" << endl << endl;

    std::string result;
    result = insertSeparators(s, "", 4, true);
    cout << "OUTSIDE: " << &result << " " << result << ", len: "<< result.length()<< ", capacity: "<< result.capacity() << " - string returned from function" << endl;
    cout << "OUTSIDE: " << &s << " " << s << ", len: "<< s.length()<< ", capacity: "<< s.capacity() << " - original string s after function" << endl << endl;

    result = insertSeparators(s, "_", 4, true);
    cout << "OUTSIDE: " << &result << " " << result << ", len: "<< result.length()<< ", capacity: "<< result.capacity() << " - string returned from function" << endl;
    cout << "OUTSIDE: " << &s << " " << s << ", len: "<< s.length()<< ", capacity: "<< s.capacity() << " - original string s after function" << endl << endl;

    result = insertSeparators(s, "_*_", 4, false);
    cout << "OUTSIDE: " << &result << " " << result << ", len: "<< result.length()<< ", capacity: "<< result.capacity() << " - string returned from function" << endl;
    cout << "OUTSIDE: " << &s << " " << s << ", len: "<< s.length()<< ", capacity: "<< s.capacity() << " - original string s after function" << endl << endl;

    result = insertSeparators(s, "_viv_", 3, true);
    cout << "OUTSIDE: " << &result << " " << result << ", len: "<< result.length()<< ", capacity: "<< result.capacity() << " - string returned from function" << endl;
    cout << "OUTSIDE: " << &s << " " << s << ", len: "<< s.length()<< ", capacity: "<< s.capacity() << " - original string s after function" << endl << endl;

    result = insertSeparators(s, "_", 11, false);
    cout << "OUTSIDE: " << &result << " " << result << ", len: "<< result.length()<< ", capacity: "<< result.capacity() << " - string returned from function" << endl;
    cout << "OUTSIDE: " << &s << " " << s << ", len: "<< s.length()<< ", capacity: "<< s.capacity() << " - original string s after function" << endl << endl;

    result = insertSeparators(s, "_", 11, true);
    cout << "OUTSIDE: " << &result << " " << result << ", len: "<< result.length()<< ", capacity: "<< result.capacity() << " - string returned from function" << endl;
    cout << "OUTSIDE: " << &s << " " << s << ", len: "<< s.length()<< ", capacity: "<< s.capacity() << " - original string s after function" << endl << endl;

    return 0;
} // int main()


/* 
// sample output of above code:
// The debugging messages show by the way how many strings are created (3 of them actually), 
// and string memory management - one of them increases its capacity, when needed.

Strings and their addresses:

OUTSIDE: 0x818c3ffb00, Hello World!, len: 12, capacity: 15 - original string s passed as arg to function

        Inside Params: src: Hello World! sep:  group len: 4 from right: 1
OUTSIDE: 0x818c3ffb20 Hello World!, len: 12, capacity: 15 - string returned from function
OUTSIDE: 0x818c3ffb00 Hello World!, len: 12, capacity: 15 - original string s after function

        Inside Params: src: Hello World! sep: _ group len: 4 from right: 1
        Inside: 0x818c3ffb00 Hello World! - src passed seen inside
        Inside: 0x818c3ffa50 Hello World! - s declared inside
        Inside: 0x818c3ffa50 Hello Wo_rld! - s after sep insertion
        Inside: 0x818c3ffa50 Hell_o Wo_rld! - s after sep insertion
OUTSIDE: 0x818c3ffb20 Hell_o Wo_rld!, len: 14, capacity: 15 - string returned from function
OUTSIDE: 0x818c3ffb00 Hello World!, len: 12, capacity: 15 - original string s after function

        Inside Params: src: Hello World! sep: _*_ group len: 4 from right: 0
        Inside: 0x818c3ffb00 Hello World! - src passed seen inside
        Inside: 0x818c3ffa50 Hello World! - s declared inside
        Inside: 0x818c3ffa50 Hello Wo_*_rld! - s after sep insertion
        Inside: 0x818c3ffa50 Hell_*_o Wo_*_rld! - s after sep insertion
OUTSIDE: 0x818c3ffb20 Hell_*_o Wo_*_rld!, len: 18, capacity: 30 - string returned from function
OUTSIDE: 0x818c3ffb00 Hello World!, len: 12, capacity: 15 - original string s after function

        Inside Params: src: Hello World! sep: _viv_ group len: 3 from right: 1
        Inside: 0x818c3ffb00 Hello World! - src passed seen inside
        Inside: 0x818c3ffa50 Hello World! - s declared inside
        Inside: 0x818c3ffa50 Hello Wor_viv_ld! - s after sep insertion
        Inside: 0x818c3ffa50 Hello _viv_Wor_viv_ld! - s after sep insertion
        Inside: 0x818c3ffa50 Hel_viv_lo _viv_Wor_viv_ld! - s after sep insertion
OUTSIDE: 0x818c3ffb20 Hel_viv_lo _viv_Wor_viv_ld!, len: 27, capacity: 30 - string returned from function
OUTSIDE: 0x818c3ffb00 Hello World!, len: 12, capacity: 15 - original string s after function

        Inside Params: src: Hello World! sep: _ group len: 11 from right: 0
        Inside: 0x818c3ffb00 Hello World! - src passed seen inside
        Inside: 0x818c3ffa50 Hello World! - s declared inside
        Inside: 0x818c3ffa50 Hello World_! - s after sep insertion
OUTSIDE: 0x818c3ffb20 Hello World_!, len: 13, capacity: 30 - string returned from function
OUTSIDE: 0x818c3ffb00 Hello World!, len: 12, capacity: 15 - original string s after function

        Inside Params: src: Hello World! sep: _ group len: 11 from right: 1
        Inside: 0x818c3ffb00 Hello World! - src passed seen inside
        Inside: 0x818c3ffa50 Hello World! - s declared inside
        Inside: 0x818c3ffa50 H_ello World! - s after sep insertion
OUTSIDE: 0x818c3ffb20 H_ello World!, len: 13, capacity: 30 - string returned from function
OUTSIDE: 0x818c3ffb00 Hello World!, len: 12, capacity: 15 - original string s after function

Process finished with exit code 0
*/