On-the-fly modifications of the links according to a set of rules (Greasekit / Javascript)

Question

I have an HTML page with loads of entries like this:

Answer 1

Something like this might work:

// the new base url
var base = ' https://www.example.co.uk/gp/wine/order?ie=UTF8&asin=';
// all the links with className 'PrmryBtnMed'
var links  = document.getElementsByTagName('a');

for(var i = 0;i < links.length;i++){
    // check each link for the 'asin' value
    var result = /asin=([\d\w]+)/.exec(links[i].getAttribute('href'));
    if(result){
        // make a new url using the 'base' and the 'asin' value
        links[i].setAttribute('href', base+result[1]);
    }
}

Demo: http://jsfiddle.net/louisbros/L8ePL/