std::basic_string<CharT,Traits,Allocator>::find_first_not_of

#include <iostream>
#include <string>
 
int main()
{
    // Permit uppercase letters, lowercase letters and numbers in macro names
    const char* pattern = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
                          "abcdefghijklmnopqrstuvwxyz"
                          "0123456789";
 
    std::string data = "1) %FIX, 2) %HACK, and 3) %TODO";
    const std::string replacement = "%DONE%";
 
    std::cout << "Before: " << data << '\n';
 
    for (std::string::size_type first{}, last{};
        (first = data.find('%', first)) != std::string::npos;
        first += replacement.size())
    {
        last = data.find_first_not_of(pattern, first + 1);
        if (last == std::string::npos)
            last = data.length();
 
        // Now first at '%' and last is one past end of the found substring
        data.replace(first, last - first, replacement);
    }
 
    std::cout << "After: " << data << '\n';
}