...
You don't need anything else than Eclipse 3.4+ version. The feature itself uses the standard java regular expressions and is actually fairly simple to understand. I would recommend to have a go at regular expressions before trying that feature. For instance, you can try http://www.gskinner.com/RegExr/ or http://www.regextester.com You may use these plugins Bastian Bergerhoff's Brosinski's to help your work.
You may also use online tools like gskinner's or regextester to test the results. Another great read is also: http://java.sun.com/developer/technicalArticles/releases/1.4regex
Be aware that there are different types of regexp. So not all commands may work (luckily, eclipse displays the available commands on the input dialog)
...
It'll grab the start and ending ending of a specified html tag. You can then use the select the inner element using $1 You may use these plugins Bastian Bergerhoff's Brosinski's to help your work.