How to extract text using sed

I have a text file and want to extract only text beginning and ending with certain strings using sed. For example, in the line:

string>![TEST[Extract this string]>/string>

I want to get

Extract this string 

How would I implement this with SED? Basically I want to get text that start with the expression "string>![TEST[" and end with the expression "]>/string>".

Thanks