>> You can use IO.pos to seek within a file, however what will you do if
>> your substitute string is not the same length as the original string?
>> Files have no concept of insert/delete.
>>
> 
> Although it would be an interesting exercise to create
> a descendant of File that would behave that way. It would
> be analogous to the way strings are reallocated in RAM.
> 
> Inefficient, certainly, but disk drives are faster than
> they were in 1980.  ;)
> 

That's it. I know that files don't have concept of insert or delete, but 
I was telling myself that maybe Ruby provided a way to abstract this 
lack and to provide a tranparent mean to modify the content of a file. 
But I can do without it ;)

Thanks all.

Ghislain