pulling custom rankings in Google Analytics

pulling custom rankings in Google Analytics

can someone explain this script to me. It is supposed to pull specific
rankings in Google however, i am unsure how this replace function is
working. What does "m," "key," and "value" come from?
Basically what I don't understand is how the ranking data is being
collected from this script.
function rankingsPush()
{
var url = String(document.referrer);
// confirm they came from G
if (url.indexOf ("google.") !=-1)
{
var urlVars = {};
var parts = url.replace(/[?&]+([^=&]+)=([^&]*)/gi, function(m,key,value)
{
urlVars[key] = value;
});
// Push to GA Custom Variables
_gaq.push(['_setCustomVar', '1', 'Rankings', urlVars["cd"], 2]);
// Return the ranking in case
// you dynamic targeting based on ranking
return urlVars["cd"];
}
}