subtle

subtle matches the WM_NAME and the WM_CLASS property of a window and not the title. This to ensure the match is correct.
There are several ways to get these values, here are the most common:

Select the window with xprop have a look for the WM_NAME and WM_CLASS lines which usually look like this:

1  WM_NAME(STRING) = "urxvt" 
2  WM_CLASS(STRING) =  "irssi",  "URxvt"

Run subtler -cl and look for the matching line:

1 0x800007 * 1 1020x374 2 1 ---  urxvt (URxvt)