query replace in marked files

[Uwe Brauer]

>>>>> "Mats" == Mats Lidell <matsl at xemacs.org> writes:

>>>>> Uwe wrote:
   Uwe> I do remember that it is possible to have strings or regexp
   Uwe> replaced in marked string using some dired enhancement, but I
   Uwe> don't remember how exactly.  Can somebody please help me

   Mats> Are you thinking about file-transformers? It works like this. Have
   Mats> dired-use-file-transformers set and then you can mark files in dired
   Mats> and do things like "echo *b" to get the base file name. See
   Mats> dired-do-shell-command for full list of transformers.

No I think it is simpler than that and I think I explained badly what
I want. 

Suppose you have 3 files 
file1 file2 file3

All of them contain several times the string foo
I want now to replace that string in all files by say foo2

How can I do that without open file1 file2 file3.

Uwe