input: Write a function to evaluate a numerical expression given as a string and return the result.
  assert eval_expr("2+3")==5
  assert eval_expr("10-4")==6
  assert eval_expr("3*5")==15
  assert eval_expr("10/2")==5.0
  assert eval_expr("7")==7
output: def eval_expr(expr):
      return eval(expr)