UTF-8 replace example.

// Source encoding: UTF-8. "π" should be a lowercase Greek pi.

#include <iterator>         // std::(begin, end)
#include <string>           // std::string
#include <string_view>      // std::string_view
#include <unordered_map>    // std::unordered_map
#include <regex>            // std::regex

namespace text {
    using   std::string, std::string_view,
            std::cmatch, std::csub_match, std::regex;

    template< class Key, class Value > using Map_ = std::unordered_map<Key, Value>;

    const Map_<string_view, string_view>  ascii_replacements    =
    {
        { "å", "aa" }, { "ä", "ae" }, { "ö", "oe" },
        { "Å", "Aa" }, { "Ä", "Ae" }, { "Ö", "Oe" }
    };

    auto nonascii_chars_regex_spec()
        -> string
    {
        string result;
        for( const auto& key_and_value : ascii_replacements ) {
            if( not result.empty() ) { result += "|"; }
            result += key_and_value.first;      // The key, i.e. a nonascii char.
        }
        return result;
    }

    const auto nonascii_chars_regex = regex( nonascii_chars_regex_spec() );

    auto to_ascii( string_view s )
        -> string
    {
        string result;
        cmatch match;
        while( regex_search( s.data(), s.data() + s.length(), match, nonascii_chars_regex )) {
            const csub_match preceding_text = match.prefix();
            result += string_view( preceding_text.first, preceding_text.length() );
            result += ascii_replacements.at( match.str() );

            const csub_match the_rest = match.suffix();
            s = string_view( the_rest.first, the_rest.length() );
        }
        const csub_match the_rest = match.suffix();
        result += string_view( the_rest.first, the_rest.length() );
        return result;
    }
}  // namespace text

#include <iostream>
#include <iomanip>
using   std::cout, std::endl, std::left, std::setw;

auto main()
    -> int
{
     const auto& swedish_text   = "Blåbär til gröten – et naturlig val!";
     const auto w = setw( 20 );
     cout << left;
     cout << w << "Original text:"      << " '" << swedish_text << "'." << endl;
     cout << w << "Known non-ASCII:"    << " '" << text::nonascii_chars_regex_spec() << "'." << endl;
     cout << w << "ASCII text:"         << " '" << text::to_ascii( swedish_text ) << "'." << endl;
}