How do i compare two strings and get the difference?

I have explained with an example of what exactly I' am looking for in my previous post. s1, s2 are the two strings to be compared and strDiff is the desired result. How is that not clear? 

I'm comparing INI file key:value pairs. So most of the time the string will be different by just one or two charecters. I want to get the difference in the two strings.