RedirectMatch and Query String

It's not possible to use RedirectMatch in this case unfortunately; the query string is not part of URL string that RewriteMatch is compared to.

The second example works because the query string the client sent is re-appended to the destination URL - so the optional match is matching nothing, the $1 replacement is an empty string, but then the client's original query string is stuck back on.

A RewriteCond check against the %{QUERY_STRING} will be needed instead.

RewriteCond %{QUERY_STRING} page=([^&]+)
RewriteRule ^/redirect\.php$ http://somewhereelse.com/%1? [R=302,L]