[Boothby]

[Trident|MSIE]+

Sorry, but this makes no sense at all, because it would even match M or MM or MMMMMMMM or in an unbelievable worst case |||||. Uuuh.
The | has no meaning between []. The chars/letters are alternatives, so you could also write:

(T|r|i|d|e|n|t|\||M|S|I|E){1,}

So, after looking into the original source codes of YUI 2 and 3 I would suggest to replace

else{a=c.match(/MSIE\s([^;]*)/);if(a&&a[1]){g.ie=e(a[1]);}

with

else{a=c.match(/MSIE ([^;]*)|Trident.*; rv:([0-9.]+)/);if(a&&(a[1]||a[2])){g.ie=e(a[1]||a[2]);}

The browser detection is very similar in both, YUI 2 and 3 and the above code is from YUI3 and then backminified by me.

untested, hope it works