Javascript: Replace characters after _

Question

I'm trying to do something which seems fairly basic, but can't seem to get it working.

I'm trying to strip the characters after the last instance of an underscore.

I have this string, for example:

www/images/10/20120412/28-696-b0b9815463e47c9371b02b7202788a75_tn.jpg

and I'm trying to strip out the 'tn.jpg' to produce:

www/images/10/20120412/28-696-b0b9815463e47c9371b02b7202788a75_

I tried doing .slice(0,-6) but that will only work for instances of _tn.jpg and not _med.jpg.

Ultimately, I'm going to be swapping in different sizes of images (_med.jpg, _full.jpg etc.) and it needs to be only after the last underscore (there might be underscores in the URL).

Any help would be greatly appreciated!

Zack