input: Write a function to evaluate a given numerical expression in string format and return the result. The function should use Python's built-in `eval` method to compute the result. If the input string is not a valid numerical expression, the function should raise an exception.
  
  def eval_expr(expr):
     return eval(expr)
  
  assert eval_expr('5 + 3 * 2') == 11
  assert eval_expr('10 / 2') == 5.0
  assert eval_expr('7 - 4') == 3
  assert eval_expr('8 * (2 + 1)') == 24
  assert eval_expr('4 ** 2') == 16
output: def eval_expr(expr):
     return eval(expr)