Compare Special String Not Valid!

Compare

package snippet;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import org.junit.Test;

public class Compare {
	
	@Test
	public  void TestCompareString() {
	   
	        @SuppressWarnings("serial")
			List<String> ids = new ArrayList<String>(){{
	            add("22");
	            add("2B");
	            add("2A");
	            add("2");
	            add("33");
	        }};
	       
	        System.out.println("---before sort---");
	        for (String string : ids) { System.out.print(string + "\t"); } 
	        
	        
	        
	        final Pattern pattern = Pattern.compile("^([0-9]*)([a-zA-Z]*)$");
	        
	        //sort
	        Collections.sort(ids, new Comparator<String>() {
	            @Override
	            public int compare(String s1, String s2) {
	                if(s1 == s2) {
	                    return 0;
	                }
	                if(s1 == null && s2 != null) {
	                    return -1;
	                }
	                if(s1 != null && s2 == null) {
	                    return 1;
	                }
	               
	                Matcher matcher1 = pattern.matcher(s1);
	                Matcher matcher2 = pattern.matcher(s2);
	                
	                String str1_num = null;
	                String str1_char = null;
	                
	                if (matcher1.find()) {
	                	str1_num = matcher1.group(1);
	                	str1_char = matcher1.group(2);
					}

					String str2_num = null;
	                String str2_char = null;
	                
	                if (matcher2.find()) {
	                	str2_num = matcher2.group(1);
	                	str2_char = matcher2.group(2);
					}
	               
	                //compare number first,then char
	                int r = str1_num.compareTo(str2_num);
	                return  r == 0 ? str1_char.compareTo(str2_char) : r;
	            }
	        });
	        
	        //print result
	        System.out.println("\r---after sort---");
	        for (String string : ids) { System.out.print(string + "\t");}
	 
	    }
}