Regular Expression - Enter A String That Is 2 - 20 Characters Long And Only Has Letters Doesn't Work

I wanted to try creating a basic expression first. I want someone to enter a string that is 2 - 20 characters long and only has letters. This is what I use so far, and it doesn't work, nothing happens at all when I run it.

Code:
// Check for a valid name.
var namePattern = new RegExp("/^[a-zA-Z]{2,20}$/");
if (namePattern.test(name))
{
window.alert("Invalid");
}

name comes from a input box I display on screen. Also, I would like to update the expression so that there can be one space in the string, but it cannot be at the beginning or end. So this string is like a persons name. Bob is OK, Bob Smith is also OK. Edit: Actually, it is doing something but its always coming back invalid.