function validate(){
    var sender_name  = document.getElementById("sender_name");
    var sender_email = document.getElementById("sender_email");
    var _regExpEmail = /^([a-z0-9_-]+[\.]?)*[a-z0-9_-]+@([a-z0-9_-]+[\.]?)*([a-z0-9_-]+)\.[a-z0-9]+$/i;
    var error        = 0;


    if( sender_name.value == ""){
        error = 1;
    }

    if( error == 0 ){
        if( sender_email.value == ''){
            error = 2;
        }
        else{
            if( !_regExpEmail.test(sender_email.value) ){
                error = 3;
            }
        }
    }

    if( error == 0 ){
        for( i=1; i<6 ; i++ ){
            receiver_name   = document.getElementById('receiver_name_' + i);
            receiver_email  = document.getElementById('receiver_email_' + i);
            if( i == 1 ){
                if( receiver_name.value == "" && receiver_email.value == "" ){
                    error = 4;
                    break;
                }
            }

            if( receiver_name.value == '' && receiver_email.value == '' ){
                break;
            }
            else{
                if( receiver_name.value == '' ){
                    error = 5;
                    break;
                }
                if( error == 0 ){
                    if( receiver_email.value == '' ){
                        error = 6;
                        break;
                    }
                    else{
                        if( !_regExpEmail.test(receiver_email.value) ){
                            error = 7;
                            break;
                        }
                    }
                }
            }


        }
    }

    if( error == 0 ){
        return true;
    }
    else{
        switch(error){
            case 1 : alert("Please enter your name!");
                break;
            case 2 : alert("Please enter your e-mail!");
                break;
            case 3 : alert("Please check your e-mail!");
                break;
            case 4 : alert("Please enter at least one friend name and e-mail!");
                break;
            case 5 : alert("Please enter friend name # " + i);
                break;
            case 6 : alert("Please enter friend e-mail # " + i);
                break;
            case 7 : alert("Please check friend e-mail # " + i);
                break;
        }
        return false;
    }

}