$re = '/[0-9]+(?= |$)/';
$str = '32132132132 32132132132 321321321323 32132132132132';
$subst = "'$0',";
$result = preg_replace($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