//check whether text-field is valid
function check_text(field, message){
	if(trim(field.value) == ""){
		alert(message);
		field.focus();
		return true;
		}
	else{
		return false;
		}
	}
//check whether text-field is valid
function check_text(field, message){
	if(trim(field.value) == ""){
		alert(message);
		field.focus();
		return true;
		}
	else{
		return false;
		}
	}
//check whether email-field ist valid
function check_email(field, message) {
    var a = false;
    var res = false;
	var email = trim(field.value);
	
    if(typeof(RegExp) == 'function'){
        var b = new RegExp('abc');
        if(b.test('abc') == true){
		    a = true;
			}
        }
    if(a == true){
        reg = new RegExp('^([a-zA-Z0-9-._]+)'+'(@)([a-zA-Z0-9-.]+)'+'(.)([a-zA-Z]{2,4})$');
 		res = (reg.test(email));
        }
    else{
        res = (email.search('@') >= 1 && email.lastIndexOf('.') > email.search('@') && email.lastIndexOf('.') >= email.length-5);
 		}
	if(res == false){
		alert(message);
		field.focus();
		return true;
		}
	else{
 		return false;
		}
	}
//check whether text-area is valid
function check_textarea(field, message){
	if(trim(field.value) == ""){
		alert(message);
		field.focus();
		return true;
		}
	else{
		return false;
		}
	}
//check whether password-field is valid
function check_password(field, message, maxlenght, minlenght){
	var pwd = trim(field.value);
	if(pwd.length > maxlenght || pwd.length < minlenght){
		alert(message);
		field.focus();
		return true;
		}
	else{
		return false;
		}
	}
//check whether file-field is valid
function check_file(field, message){
	if(trim(field.value) == ""){
		alert(message);
		field.focus();
		return true;
		}
	else{
		return false;
		}
	}
//check whether select-field is valid
function check_select(field, message){
	if (field.selectedIndex == 0) {
		alert(message);
		field.focus();
		return true;
		}
	else{
		return false;
		}
	}
//check whether radio-field is valid
function check_radio(field, message){
    for(var i=0; i<field.length; i++){
		if(field[i].checked){
			return false;
			}
		}
	alert(message);
	field.focus();
	return true;
	}
//check whether checkbox-field is valid
function check_checkbox(field, message){
    for(var i=0; i<field.length; i++){
		if(field[i].checked){
			return false;
			}
		}
	alert(message);
	field.focus();
	return true;
	}
//trim string
function trim(str){
	while(str.substring(0,1)==' '){
		str=str.substring(1,str.length);
		}	
	while(str.substring(str.length-1,str.length)==' '){
		str=str.substring(0,str.length-1);
		}
	return str;
	}