JS 正则式问题

Question

window.onload = function () {
            var replaceMsg = "wHENEVER SANG MY SONG,oN THIS STAGE<BR /> &COPY;1999-2006";
            var matchExp = /([\d])|([a-z])|([A-Z])/g;
            document.body.innerHTML = replaceMsg.replace(matchExp, magicCode);
        }

 

        function magicCode(s, a, b, c) {
            return a ? "<font color='red'>" + s + "</font>" : (b ? b.toUpperCase() : c.toLowerCase());
        }

 

麻烦大侠给我解释下这段正则式什么意思啊。