gistfile1.js

'wkjkjd fsk fsdjk\nbbb sss fasdf'.replace(/(bbb|sss)(\s)?/g, function(match, p1, p2){
    var min = 1, max = p1.length - 1,
        pos = Math.floor(Math.random() * (max - min + 1)) + min; //randint
    return p1.substr(0, pos) + 'a' + p1.substr(pos, p1.length - 1) + p2;
});