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I want to replace multiple matches of a Regex with different values from a map.

I have for example the following string #id#_#date#_#value#_additional_text.

I now want to replace the parts #xxx# with the corresponding values from a map. (The string could change so that I don't exactly know what kind of patterns are in there.

What I'm currently doing are the following steps:

  1. Use std::sregex_token_iterator to go through the string and store all patterns I found in a std::vector.
  2. Go through all the patterns in the vector and use std::regex_replace to replace them with the values from the map.

This is the code for the steps above:

int main() {
    std::map<std::string, std::string> metadata{
        {"value", "9"},
        {"id", "1234"},
        {"date", "1234"},
        {"more", "abc"}};
    std::vector<std::string> patterns{};
    std::string input_data = "#id#_#date#_#value#_additional_text";
    std::regex reg{R"(#([a-zA-Z]+)#)"}; 

    const std::sregex_token_iterator end;
    for (std::sregex_token_iterator iter{std::cbegin(input_data), std::cend(input_data), reg, 1}; iter != end; ++iter) {
        std::cout << iter->str() << '\n';
        patterns.push_back(iter->str());
    }

    for (const auto pattern : patterns) {
        std::cout << pattern << '\n';
        std::regex regex_pattern{"#" + pattern + "#"};
        input_data = std::regex_replace(input_data, regex_pattern, metadata[pattern]);
        std::cout << input_data << '\n';
    }
    std::cout << input_data << '\n';
}

My question now is, is there a better way to achieve this?

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  • \$\begingroup\$ There are several questions for edge case, such as, what happens if replaced value is of the form #xxx# (and so might be replaced by next replacement). \$\endgroup\$
    – Jarod42
    Jan 17, 2022 at 13:31
  • \$\begingroup\$ You currently replace unknown patterns by nothing, is it intended? \$\endgroup\$
    – Jarod42
    Jan 17, 2022 at 13:32

1 Answer 1

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Library includes

The code doesn't compile as presented. I needed to add a few headers:

#include <iostream>
#include <map>
#include <regex>
#include <string>
#include <vector>

Unnecessary output

The problem statement just refers to replacing parts of the string, but we seem to be writing lots of other things to std::cout:

    std::cout << iter->str() << '\n';
    std::cout << pattern << '\n';
    std::cout << input_data << '\n';

These look like leftover debugging prints (that would normally go to std::clog rather than std::cout, and be removed before the program is ready for review).

I'm assuming this output is not required.


Unnecessary copying

We don't need to copy each pattern here:

    for (const auto pattern : patterns) {

Instead, we can just bind a reference:

    for (const auto& pattern : patterns) {

Consider a single-pass algorithm without regular expressions

Since # acts as a delimiter, we can implement these substitutions much more simply - just search for # and then look to see if it's followed by one of our translation keys and another #. That would look something like this:

#include <iostream>
#include <map>
#include <string>
#include <string_view>

std::string replace_in_string(std::string_view s,
                              const std::map<std::string, std::string>& replacements)
{
    std::string result;

    for (;;) {
        auto pos = s.find('#');
        auto end = s.find('#', pos+1);
        if (end == std::string_view::npos) {
            return result.append(s);
        }
        auto const key = s.substr(pos+1, end - pos - 1);
        auto const it = replacements.find(std::string{key});
        if (it != replacements.end()) {
            result.append(s.substr(0, pos)).append(it->second);
            s = s.substr(end+1);
        } else {
            result.append(s.substr(0, end));
            s = s.substr(end);
        }
    }
}

int main() {
    const std::map<std::string, std::string> metadata{
        {"value", "9"},
        {"id", "1234"},
        {"date", "1234"},
        {"more", "abc"}};
    const std::string input_data = "#id#_#date#_#value#_additional_text";

    std::cout << replace_in_string(input_data, metadata) << '\n';
}
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