function nthReplace(str, regex, newValue, n) { 
    return str.replace(regex, function(a) { return (--n == 0) ? newValue : a; }); 
}

alert(nthReplace("content 1 content 2 content 3", /content/g, "****", 2));

var sub = 'content';
var str = '<meta http-equiv="content-type" content="text/html; charset=utf-8" />';
var rep = '%%5%%';
var index = 32;

str = str_replace(str, sub, rep, index)
alert(str)

function str_replace(str, sub, rep, index){
	str = str.split('');
	str.splice(index, sub.length, rep);
	return str.join('');
}

var str = '<meta http-equiv="content-type" content="text/html; charset=utf-8" />';
str = str.replace(/(.+)content="(.+);(.+)/, '$1content="ololo;$3');
alert(str);