comment form custom validation

Question

I am using a custom validation in my Drupal 7 comment form.
I want to match the comment_body against a word "example" so if someone just type "example"in the comment body then it will show a error message that only the word "example" is not allowed. After reading drupal api documentation i codded like this:

function mango_form_comment_node_news_form_alter(&$form, &$form_state, $form_id){
    $form['comment_body']['#prefix'] = '<div class="sm">Post your comment now!</div>';
    $form['actions']['submit']['#value'] = 'Post Your Comment';
    $form['#validate'][] = 'banana';
}

function banana($form, &$form_state) {
  if ($form_state['comment_body'] == "example") {
    form_set_error('comment_body', 'Your comment can not contain example!');
  }
}

Do not working :'( what i'm missing? Thanks

Answer 1

The values for comment_body is not stored in $form_state but in something like:

$form_state['values']['comment_body']['und'][0]['value']

function banana($form, &$form_state) {
  if ($form_state['values']['comment_body']['und'][0]['value'] == "example") {
    form_set_error('comment_body', 'Your comment can not contain example!');
  }
}