input: Write a function to evaluate a given numerical expression in string format and return the result of the evaluation.
  
  def eval_expr(expr):
     return eval(expr)
  
  assert eval_expr("2+2")==4
  assert eval_expr("10-5")==5
  assert eval_expr("3*4")==12
  assert eval_expr("15/3")==5.0
  assert eval_expr("0")==0
output: def eval_expr(expr):
      return eval(expr)