$re = '/HTTP\S+(\s,|$)/';
$str = 'Lorem ipsum dolor sit amet, consectetur adipiscing elit, HTTP://unknown/string/in/unknown/place/ , sed do eiusmod tempor incididunt ut labore et dolore magna aliqua
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua, HTTP://unknown/string/in/unknown/place/
';
$subst = "";
$result = preg_replace($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