Valeria, while you are taking a look at A Better rename have a look at A.F.5 Rename.
it is a free program, I use it in my studio, it does a great job. This is obvious a renaming program you would use after lightroom.
It can quickly and easily rename a whole folder. You also get to see the end result of the rename before you commit the changes.
It can do what you are requesting in seconds!
You can download it from http://www.fauland.com/af5.htm