$re = '/(?:.+?)(.{6}<em>.+?<\/em>.{6})(?:.+)/m';
$str = 'шла Саша по шоссе и <em>сосала</em> сушку, или что-то другое';
$subst = "...$1...";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php