How do get year in string in SQL server 2005 ?

hi
I have 1 string : 20/05/2011
I want get year in this string on 2 case : 
 
case 1 : select year(getdate()) case 2 : select convert(varchar(10),year(20/05/2011),100)  
Result : 2011. But it`s not worked
You can help me ?
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