how i can check or validated the text field which i declare in magento system.xml i mean when i enter the word in text field and press save config button the it will show message "this is reserved word". where i can call or set this function for system.xml text field.over code is.
public function isReserveWord($word) {
$is_reserved = false;
$reserve_words = array('supportportal','view','search','askquestion');
if (in_array($word,$reserve_words)) {
$is_reserved = true;}
return $is_reserved;}
yes you can manage with system.xml to make you own form in your extension
here i can give you very good document link which will very useful to you
http://alanstorm.com/magento_system_config_validation
just understand carefully and you can definitely achieve this by server side validation.
hope this will sure help you.