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
  assert eval_expr("(4+6)*3")==30
  assert eval_expr("0")==0
output: def eval_expr(expr):
     return eval(expr)