Test For Alphanumeric Expressions In Input Field?

I have a form and I need to validate a field against three rules:

1) The field need to be between 6 and 12 characters
2) It can only have letters, numbers, and the underscore
3) It cannot contain a space or other special characters

I want the validate to happen in real-time. I have the first rule working great. Here is the code for that:

var username = document.getElementById('registerUsername');
if((username.value.length < 6) || (username.value.length > 12))
{
document.getElementById('usernameValidate').innerHTML="Incorrect.";

[Code]....