how can i get the particular post id in foreach loop

Is there any chance to submit the comment in ajax 

Sure, in order to do that, first alter this line:

PHP Code:

echo form_input('post_comment[]',NULL,'size="40" class="ta-comment"'); 


 into:

PHP Code:

echo form_input('post_comment[]',NULL,'size="40" class="ta-comment" postid="' . $row->post_id . '"'); 


Then, in the script section, make the following changes:

Code:

<script>

$('.ta-comment').keypress(function(event) {
  if (event.which == 13 ) {
    event.preventDefault();
    url = "<?= base_url();?>regcheck/ajax_comment_insert";
    ctext = $(this).val();
    postid = $(this).attr('postid');
    $.ajax({

      url : url,
      type: "POST",
      data : { comment: ctext, post_id: postid }
    });
  } 
  
});  

</script>

And finally, you need to add a new method to your controller Regcheck, to make it insert the posted comment:

PHP Code:

public function ajax_comment_insert()
{
  $data = array(
   'post_id' => $this->input->post('post_id'),
   'comment' => $this->input->post('comment')
  );
  $this->db->insert('post_comments',$data);
}