I am referring to this feature:
Note how Stack Overflow shows which bits have been deleted, and which parts have been added (shown in red and green).
Basically, I was wondering if Stack Overflow used an external piece of software for this capability. If they did not, could I know how they implemented such a thing (and how I could implement it in PHP)?
The main reason I think this is a plugin / external piece of software is because I was reading a post stating that the Stack Overflow live preview editor was made using WMD Markdown Editor.