The Modern C++ Challenge

31. Transforming dates in strings

Text transformation can be performed with regular expressions using std::regex_replace(). A regular expression that can match dates with the specified formats is (\d{1,2})(\.|-|/)(\d{1,2})(\.|-|/)(\d{4}). This regex defines five capture groups; the 1^st is for the day, the 2^nd is for the separator (. or -), the 3^rd is for the month, the 4^th is again for the separator (. or -), and the 5^th is for the year.

Since we want to transform dates from the format dd.mm.yyyy or dd-mm-yyyy to yyyy-mm-dd, the regex replacement format string for std::regex_replace() should be "($5-$3-$1)":

std::string transform_date(std::string_view text){   auto rx = std::regex{ R"((\d{1,2})(\.|-|/)(\d{1,2})(\.|-|/)(\d{4}))" }; return std::regex_replace(text.data(), ...