$re = '/^([0-9]*)([A-Z]?),((?:(?!\sAPP|\sAPT).)*)(?:\sAPP|\sAPT)?(.*)$/mi';
$str = '5847A, rue Principal APP A
5847A, rue Prince Arthur APT 22
1111, Sherwood street';
$subst = "$1,$2,$3,$4";
$result = preg_replace($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