$re = '/<loc>(.+)<\/loc>.*2020-08-06/';
$str = '<?xml version=\'1.0\' encoding=\'UTF-8\'?><urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<url><loc>https://google.com/2020/08/this1.html</loc><lastmod>2020-08-06T11:30:55Z</lastmod></url>
<url><loc>https://google.com/2020/08/this2.html</loc><lastmod>2020-08-05T11:30:06Z</lastmod></url>
<url><loc>https://google.com/2020/08/this3.html</loc><lastmod>2020-08-06T11:29:25Z</lastmod></url>
</lastmod></url></urlset>';
$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