#include <string>
#include <iostream>
 
int main() {
    std::string to_search = "Some data with %MACROS to substitute";
    std::string replace_with = "some very nice macros";
    std::string macro_name;
 
    std::cout << "Before: " << to_search << '\n';
 
    std::string::size_type pos = 0;
    while ((pos = to_search.find('%', pos)) != std::string::npos)
    {
        // Permit uppercase letters, lowercase letters and numbers in macro names
        auto after = to_search.find_first_not_of("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789", pos + 1);
 
        // if no matching chars, set after past the last char...
        if (after == std::string::npos)
            after = to_search.size();
 
        auto count = after - pos;
        if (count > 1)
        {
            // extract the macro name and replace the macor only if
            // the name actually maps to something worth replacing...
 
            macro_name = to_search.substr(pos + 1, count - 1);
            if (macro_name == "MACROS")
            {
                // found a macro!
                to_search.replace(pos, count, replace_with);
 
                // start the next search after the replaced text...
                pos += replace_with.size();
 
                continue;
            }
 
            // check for other macros as needed...
        }
 
        // no matching macro, skip the '%' and search again from the next char...
        ++pos;
    }
 
    std::cout << "After: " << to_search << '\n';
}

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