RegExp question: how to add a number?

Hi everyone, I hope you're doing well (haven't written here for a long time :-)). The question is as follows: I have a regexp that would do the following. If the string begins with "Re:", it will change the beginning to "Re[2]:"; if it doesn't, then it would add "Re:" at the beginning. But (attention, here it is!) if the string starts with something like "Re[4]:", it should replace it by "Re[5]:". Here's the code: $start=mb_strtolower(mb_substr($f['Subject'], 0, 3)); if ($start=="re:") { $subject=preg_replace("/^re:(.+?)$/usi", "re[2]:$1", $f['Subject']); } elseif ($start=="re[") { // Here $1+1 doesn't work, it returns "Re[4+1]:"! $subject=preg_replace("/^re\[(\d+)\]:(.+?)$/usi", "re[$1+1]:$2", $f['Subject']); } else { $subject="Re: ".$f['Subject']; } I know there actually exists a way to do the numeral addition ("Re[5]:", not "Re[4+1]:"). How do I manage to do this? Thanks! -- With best regards from Ukraine, Andre Skype: Francophile Twitter: http://twitter.com/m_elensule Facebook: http://facebook.com/menelion