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e=document.getElementsByTagName("TD");  
for(k=15; k<e.length;k++) {  
if(e[k].className=='posttdMessage') {  
s=e[k].innerHTML;  
while (s.indexOf('[v ideo]') != -1){  
s=s.replace('[v ideo]','<fieldset>'); s=s.replace('[/v ideo]','</fieldset>');  
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<input type="button" value="Видео" onclick="simpletag('video','','','message',''); return false;" class="codeButtons" title="Вставить Видео" name="video"/>