javascript regex (username validation)

I want to enforce that

1. the input firstname should only contains characters A-Z, a-z, and -
2. the input login name should only contains alphanumeric characters

How do I restrict the two rules in javascript?

Below is my code (jsp) for username regex. But it's not working properly.

function validateForm(){
    var nameRegex = /^[a-zA-Z\-]+$/;
    var validfirstUsername = document.frm.firstName.value.match(nameRegex);
    if(validUsername == null){
        alert("Your first name is not valid. Only characters A-Z, a-z and '-' are  acceptable.");
        document.frm.firstName.focus();
        return false;
    }
}

Thanks!