only matches surrounded by word boundaries.
In TO-STRING, `\\&' stands for whatever matched the whole of REGEXP,
and `\\=\\N' (where N is a digit) stands for
- whatever what matched the Nth `\\(...\\)' in REGEXP."
+ whatever what matched the Nth `\\(...\\)' in REGEXP.
This function is usually the wrong thing to use in a Lisp program.
What you probably want is a loop like this: