I don't think that backslash should be escaped.

    MSIE ([0-9]{1,}[\\.0-9]{0,})

![Regular expression visualization](https://www.debuggex.com/i/V6175NjAp8-cerZu.png)

[Debuggex Demo](https://www.debuggex.com/r/V6175NjAp8-cerZu)

I'd suggest this regexp literal instead:

    /MSIE ([0-9]+\.[0-9]+)/

It matches [all known UA Strings](http://useragentstring.com/pages/Internet%20Explorer/) for IE 2.0-10.6, though it omits the beta tag from versions like `5.0b1`.