Regex calendar validation

I am trying to make a program where the user enters a date(any date), for example 29-Jul-1995. The program should match the date format and validate if this date is correct or not.

Here is my code:

import java.util.*;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Review1_Main {

public static Pattern pattern;
public static Matcher matcher;

// DATE_PATTERN is a format for user input
private static final String DATE_PATTERN = "(0?[1-9]|[12][0-9]|3[01])-(^[a-zA-Z]+$)-((19|20)\\d\\d)";

public Review1_Main() {
   // Applying DATE_PATTERN format to pattern
   pattern = Pattern.compile(DATE_PATTERN);
}

// Date format validation
public static boolean dateValidation(String date) {

matcher = pattern.matcher(date);

if(matcher.matches()) {
  matcher.reset();

  if (matcher.find()) {
    int day = 01;
    String month = "Jan";
    int year = 2000;

    if(day == 31 && !month.equals("Apr") || !month.equals("Jun") ||
                    !month.equals("Sep") || !month.equals("Nov")) {
      return false;
    } else if(month.equals("Feb")) {
      // LEAP year validation
      if (year % 4 == 0) {
        if (day == 30 || day == 31) {
          return false;
        } else {
          return true;
        }
      } else {
        if (day == 29 || day == 30 || day == 31) {
          return false;
        } else {
          return true;
        }
      }// end of year % 4
    } else {
      return true;
    }// end of day.equals
  } else {
    return false;
  } // end of matcher.find
} else {
  return false;
}// end of matcher.matcher

}

 public static void main(String[] args) {
   Scanner s = new Scanner(System.in);
   String dateInput = "29-Jul-1995";
   dateValidation(dateInput);
 }

} // end of class

But for some reason, when I try to compile it, I always get this error:

Exception in thread "main" java.lang.NullPointerException
    at Review1_Main.dateValidation(Review1_Main.java:21)
    at Review1_Main.main(Review1_Main.java:72)

My apologize about my grammar.

Thank you.