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

auto date_to_yyyymmdd(std::string text0)
{
  auto rex1 = R"((\d{1,2})(\.\s|[\.|/|\-])(\d{1,2})(\.\s|[\.|/|\-])(\d{4}))";
  auto fmt1 = "$5-$3-$1";

  auto rex2 = R"((\d{4})-(\d{1})-(\d{1,2}))";
  auto fmt2 = "$1-0$2-$3";

  auto rex3 = R"((\d{4})-(\d{2})-(\d{1})($|\D))";
  auto fmt3 = "$1-$2-0$3$4";

  auto text1 = std::regex_replace(text0, std::regex{rex1}, fmt1); // order by y-m-d
  auto text2 = std::regex_replace(text1, std::regex{rex2}, fmt2); // add starting zero, if one digit month
  auto text3 = std::regex_replace(text2, std::regex{rex3}, fmt3); // add starting zero, if one digit day
  return text3;
}

int main()
{
  std::cout << date_to_yyyymmdd("Datum: 29. 03. 2019") << '\n' ;
  std::cout << date_to_yyyymmdd("Datum: 29. 3. 2019") << '\n' ;
  std::cout << date_to_yyyymmdd("Datum: 29.3.2019") << '\n' ;
  std::cout << date_to_yyyymmdd("Datum: 29.03.2019") << '\n' ;
  std::cout << date_to_yyyymmdd("Datum: 29-03-2019") << '\n' ;  
  std::cout << date_to_yyyymmdd("Datum: 29/03/2019") << '\n' ;
  std::cout << date_to_yyyymmdd("Datum: 29/3/2019") << '\n' ;
  std::cout << date_to_yyyymmdd("Datum: 2. 3. 2019") << '\n' ;
  std::cout << date_to_yyyymmdd("Datum: 2.3.2019") << '\n' ;
  std::cout << date_to_yyyymmdd("Datum: 2-3-2019") << '\n' ;
  std::cout << date_to_yyyymmdd("Datum: 2/3/2019") << '\n' ;
  std::cout << date_to_yyyymmdd("Datum: 3/12/2019") << '\n' ;
}