<?php
$exp = '/[\s\S]*\{%[\s\S]*for[\s\S]*ad[\s\S]*in[\s\S]*ads[\s\S]*%\}/';
$string = "{% for ad in ads %} sdfksjf {% for ad in ads %} sdfksjf2 {% for ad in ads %} sdfksjf3";
function str_lreplace($search, $replace, $subject)
{
return preg_replace('~(.*)' . preg_quote($search, '~') . '~', '$1' . $replace, $subject, 1);
}
echo str_lreplace($exp,'',$string);