$(document).ready(function() {
   $('#c_mes').one("focus", function() {
     $('#c_mes').parent().after('<p id="comment-by">Live Comment Preview：↓にコメントのプレビューが表示されます。</p><div class="preview-box"><p id="live-preview"></p></div>');
   });
   
   var $c_mes = ''; // that's two single quotation-marks at the end
   $('#c_mes').keyup(function() {
     $c_mes = $(this).val();
     $c_mes = $c_mes.replace(/\n/g, "<br />")
       .replace(/\n\n+/g, '<br /><br />');
     $('#live-preview').html( $c_mes );
   });
 });