var str = 'Add 1/4 cup of sugar and 1/2 cup of blueberries.'
str.replace(/(\d+)\/(\d+)/g, function(match, submatch1, submatch2) {
    var num1 = parseInt(submatch1);
    var num2 = parseInt(submatch2);
    return (num1 / num2).toString();
});
// "Add 0.25 cup of sugar and 0.5 cup of blueberries."