It doesn't work for you because the way you wrote the RegExp. It will return an array of matches.
Your code should be like this
code:
var text = Message.match(/((\[b\](.*?)\[\/b\](.*?))|(\[u\](.*?)\[\/u\]))/g);
This should work as well
code:
var sMessage = "[n]stuff[/n] hey wow [n]more stuff[/n]";
var NoParse = sMessage.match(/\[n\].+?\[\/n\]/mgi);
Your results would be
NoParse[0] = [n]stuff[/n]
NoParse[1] = [n]more stuff[/n]