使用WPF双向数据绑定时更改setter属性中的值

I have a TextBox that is bound to a Text-property on an Entity-object. I'd like to be able to re-format the text the user enters in some circumstances - e.g. if the user enters "2/4" (a fraction) - I'd like to change that to "1/2".I have a TextBox that is bound to a Text-proper