replace-regexp question

[Neon Absentius <absent@sdf.lonestar.org>]

Is there a way to use the command replace-regexp to replace a number with 
that number increased, say, by 15?  I tried 

C-M-% -?[0-9]+ RET (number-to-string (+ (string-to-number "\&") 15)) RET

but it doesn't work because emacs interprets the second input to be a string 
and so it doesn't evaluate it.  So I guess the question is how can we use
the result of evaluating a sexp for replacement text?

Any help and/or pointers to documentation will be greatly appreciated. 

-- 
 There is no national science just as there is no national
multiplication table; what is national is no longer science.
         -- Anton Checov