function validate(){
    var first_name = document.getElementById("first_name");
    var last_name  = document.getElementById("last_name");
    var email      = document.getElementById("email");
    var message    = document.getElementById("message");
    var _RegExp    = /^([a-z0-9_-]+[\.]?)*[a-z0-9_-]+@([a-z0-9_-]+[\.]?)*([a-z0-9_-]+)\.[a-z0-9]+$/i;
    var error      = 0;


    if( first_name.value == '' ){
        error = 1;
    }
    if( error == 0 && last_name.value == '' ){
        error = 2;
    }
    if( error == 0 && message.value == '' ){
        error = 3;
    }

    if( error == 0 && email.value == '' ){
        error = 4;
    }
    else if( error == 0 && !_RegExp.test(email.value)){
        error = 5;
    }

    if( error != 0 ){
        switch(error){
            case 1: alert("Please enter your first name!");
                break;
            case 2: alert("Please enter your last name!");
                break;
            case 3: alert("Please enter your message!");
                break;
            case 4: alert("Please enter your e-mail address!");
                break;
            case 5: alert("Wrong e-mail format!");
                break;
        }

        return false;
    }
    else{
        return true;
    }

}