input: Write a function to evaluate a given string containing a numerical expression and return the result.
  assert eval_expr("1+1")==2
  assert eval_expr("10-5")==5
  assert eval_expr("3*4")==12
  assert eval_expr("100/2")==50
  assert eval_expr("2**3")==8
output: def eval_expr(expression):
      return eval(expression)