$re = '/\.(?!\s|$)/';
$str = 'What is the clean way in python to do this simple text fixing - checking if every full stop (except the last one) is followed by space.Assume that having a dot not followed by an empty space is the only possible error we can get in the input string.';
$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