[e1f]

Kolyaj,
блин, ступил. Так:

var reg = /(page=)(\d+)(\&url)/;
var qwer = "http://site.ru/index.php?page=589&url=http%3A%2F%2Fdomen.ru%2F";
qwer = qwer.replace(reg, function(all, one, two, three){return one+((Number(two)||0)+1)+three});