var reg = /page=(\d+)\&url/;
var qwer = "http://site.ru/index.php?page=589&url=http%3A%2F%2Fdomen.ru%2F";
var arrq = reg.exec(qwer);
var ert = Number(arrq[1]) 
alert(qwer.replace(arrq[1],ert+1));