Challenge :: Differentiate Polynomials

Question

This challenge is about to differentiate a polynomial.
(hope you know how to do it, otherwise you can consult here:
http://www.wikihow.com/Differentiate-Polynomials )

input : polynomial string in the form "a*x**n + b*x**(n-1) + ... + c"
-all coefficients are integer
-coefficients can be positive or negative
-the first coefficient can be without leading "+", e.g. ( "2*x .. instead of "+2*x..)
-all exponents (powers) are positive integer
-polynomial expression is valid (for evaluation)
-all "1" are skipped, e.g. 1*x is x, x**1 is x
output : the derivative
example: function ("x**2+2*x+1") returns "2*x+2"

(PS. practice regex or use your own way).

Answer 1

is it too hard?
Test cases:
"4*x**2+2*x+1"
"x+1+x**2"
"-1000*x**1024+x-17"
"-x+1"
"1"
"0"