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you can learn a lot about regexp starting from here.

http://w3schools.com/js/js_obj_regexp.asp

use the try it editor for quick preview of what exactly happens. its really not as hard as you may think.

note: i haven't used the addon, i'm just offering help on regexp for all the people that miss this addon. i like the said features of this addon. the idea of previewing redirection urls and using it preview any short url redirection is really great.