How to give Error message when user input wrong format of date in Java8

Question

I want to give an error message if the user inputs using the wrong format. The correct format is "yyyy-MM-dd HH:mm:ss". How can I put that as a condition?

for example if (yyyy <0 ) { sout("please input correct year")}

this is the code i use for ask the user and formatting it

Scanner keyboard = new Scanner(System.in);
        String hour = "00:00:00";
        System.out.println("Please enter Date : ");
        String time = keyboard.next()+" "+ hour;
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
        LocalDateTime dateTime = LocalDateTime.parse(time, formatter);

Answer 1

You can use a regex comparison:

while input doesn't match the regex pattern
     print "Please enter date in the correct format: yyyy-MM-dd HH:mm:ss"
continue with the rest of the code

The RegEx pattern could be:

\d{4}-[01]\d-[0-3]\d [0-2]\d:[0-5]\d:[0-5]\d(?:.\d+)?Z?

You can use this site to create and test RegEx patterns