input: Write a function to evaluate a given string containing a numerical expression and return the result.
  def eval_expr(expr):
     return eval(expr)
  assert eval_expr("2+3")==5
  assert eval_expr("5*6")==30
  assert eval_expr("10/2")==5.0
  assert eval_expr("4**2")==16
  assert eval_expr("(3+4)*2")==14
output: def eval_expr(expr):
     return eval(expr)