$re = '/([\d:]*)\s*GMT/im';
$str = 'located on 10/6/2015 5:18:14 GMT';
$subst = "$1";
$result = preg_replace($re, $subst, $str, 1);
echo "The result of the substitution is ".$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