Replacing RegExp to another RegExp (JQuery, JavaScript) : **Problem**<hr/>

I have a String 

    var str = "string";

I want it will be catched with this Expression `/(^[a-z])/` and will be replaced by This Expression `/([A-Z])/`

> It means it will make the first `LowerCase` Letter to `UpperCase`





**I have tried**<hr/>

> I have tried it my self but got an unexpected result. My code was like that.

    "string".replace(/(^[a-z])/, /([A-Z])/);

> But the results of it is `/([A-Z])/` . It catches the first `Charecter` and Replace it to /([A-Z])/ as a text 