Rexx, Java - rextester

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//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_111

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

class Rextester
{  
    public static void main(String args[])
    {
        final String regex = "Diagnostic-Code[\\s\\S]*?[\\r\\n]{2}";
        final String string = "...\n"
                             + "Status: 5.0.0\n"
                             + "Diagnostic-Code: X-Postfix; skalio.com\n"
                             + "*this*\n"
                             + "*should*\n"
                             + "*be included too*\n\n"
                             + "--EA7634814EFB9.1516804532/mail.example.com\n"
                             + "Content-Description: Undelivered Message\n"
                             + "...\n\n"
                             + "Action: failed\n"
                             + "Status: 5.1.1\n"
                             + "Remote-MTA: dns; gmail-smtp-in.l.google.com\n"
                             + "Diagnostic-Code: smtp; 550-5.1.1 The email account that you tried to reach does\n"
                             + "    not exist. Please try 550-5.1.1 double-checking the recipient's email\n"
                             + "    address for typos or 550-5.1.1 unnecessary spaces. Learn more at 550 5.1.1\n"
                             + "    https://support.google.com/mail/?p=NoSuchUser u11si15276978wru.314 - gsmtp\n\n"
                             + "--E8A363093CEC.1520529178/proxy03.hostname.net\n"
                             + "Content-Description: Undelivered Message\n"
                             + "Content-Type: message/rfc822\n\n"
                             + "Return-Path: <no-reply@hostname.net>";

final Pattern pattern = Pattern.compile(regex);
        final Matcher matcher = pattern.matcher(string);

while (matcher.find()) 
        {
            System.out.println("Full match: " + matcher.group(0));
            for (int i = 1; i <= matcher.groupCount(); i++)
            {
                System.out.println("Group " + i + ": " + matcher.group(i));
            }
        }
    }
}

[ + ] Show input
Compilation time: 1.76 sec, absolute running time: 0.36 sec, cpu time: 0.37 sec, memory peak: 18 Mb, absolute service time: 2,13 sec