$re = '/#(<!--[TAG_START]-->)(.*)(<!--[TAG_END]-->)/';
$str = '<!--[TAG_START]-->a<!--[TAG_END]-->
<!--[TAG_START]-->b<!--[TAG_END]-->
<!--[TAG_START]-->c<!--[TAG_END]-->
<!--[TAG_START]-->d<!--[TAG_END]-->
<!--[TAG_START]-->e<!--[TAG_END]-->
';
$subst = "f";
$result = preg_replace($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