import java.time.*;
import java.time.format.*;
import java.util.regex.Pattern;
import java.util.regex.Matcher;
public class Main
{
public static void main(String[] args) {
LocalDate dt = LocalDate.now();
final String regex = "\"buyDate\":\"(\\d{4}-\\d{2}-\\d{2})T";
final String string = "\"buyDate\":\"2019-04-09T13:21:00.866Z\"";
DateTimeFormatter formatDateToday=DateTimeFormatter.ofPattern("YYYY-MM-dd");
final String subst = "\"buyDate\":\""+ dt.format(formatDateToday) +"T";
final Pattern pattern = Pattern.compile(regex, Pattern.MULTILINE);
final Matcher matcher = pattern.matcher(string);
// The substituted value will be contained in the result variable
final String result = matcher.replaceAll(subst);
System.out.println(result);
}
}
