<?php
$comment = '
<!--This is a comment. <!--This is a comment. Comments are not displayed in the browser --><!--This is a comment. Comments are not displayed in the browser--> Comments are not displayed in the browser-->';
preg_match('/(\<!--\s*.*?((--\>)|$))/imsx', $comment, $matches);
var_dump($matches);
array(4) {
[0]=>
string(91) "<!--This is a comment. <!--This is a comment. Comments are not displayed in the browser -->"
[1]=>
string(91) "<!--This is a comment. <!--This is a comment. Comments are not displayed in the browser -->"
[2]=>
string(3) "-->"
[3]=>
string(3) "-->"
}