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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>');    
e[k].innerHTML=s;    
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} }    
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<input type="button" value="Видео" onclick="simpletag('video','','','message',''); return false;" class="codeButtons" title="Вставить Видео" name="video"/>