help htaccess isapi rewrite

RewriteRule ^olddomain.php?cat=39$ http://www.newdomain.com/firstdir/seconddir/? [R=301,NC]
RewriteRule ^olddomain.php\?cat\=39$ http://www.newdomain.com/firstdir/seconddir/? [R=301,NC] 
RewriteRule ^olddomain.php?cat\=39$ http://www.newdomain.com/firstdir/seconddir/? [R=301,NC]
RewriteRule ^olddom开发者_如何学Cain.php\?cat=39$ http://www.newdomain.com/firstdir/seconddir/? [R=301,NC]

why they work??? the issue i think has something to do with the first question mark, the first equal sign, or the digits '39'...haha

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Remember RewriteRule matches only your URI and it will never match the host name or query string. Here is the right way to do what you are trying to do:

Options +FollowSymlinks -MultiViews
RewriteEngine on

RewriteCond %{QUERY_STRING} ^cat=(.*)$
RewriteRule ^olddomain\.php$ http://www.newdomain.com/firstdir/seconddir/? [L,R=301,NC,NE]