Filtering string using regex in utf8 format

I am trying to filter strings that escapes special characters and transforms it into lowercase. For example: "Good morning!" is transformed into good morning. I am passing one string at the time to my function. I am successfully filtering my strings that are in English language but I have problems when I am passing strings that are in my native language. What type of regex filter string should I use if I want to include all utf-8 characters?

#include <string>
#include <iostream>
#include <regex>
#include <algorithm>

std::string process(std::string s) {
    std::string st;
    std::regex r(R"([^\W_]+(?:['_-][^\W_]+)*)");
    std::sregex_iterator i = std::sregex_iterator(s.begin(), s.end(), r);
    std::smatch m = *i;
    st = m.str();
    std::transform(st.begin(), st.end(), st.begin(), ::tolower);
    return st;
}

int main() {
    std::string st = "ąžuolas!";
    std::cout << process(st) << std::endl; // <- gives: uolas
    return 0;
}