I encountered a strange error on using Java Regular Expression recently: 

```
System.out.println(Pattern.matches("(\\d\\d)", "12"));  -->  true  
System.out.println(Pattern.matches("(\\d\\d)", "11"));  -->  true  
System.out.println(Pattern.matches("(\\d\\d)\\1", "1212"));  -->  true  
System.out.println(Pattern.matches("(\\d\\d)\\1", "1122"));  -->  false  

```

What's wrong with the last one? Can anyone help me, thanks a lot~