how to compare data-group values

The log message seems to be from an earlier incarnation of the iRule since the message seems to complain about the fact that there is a dash in the variable name which is interpreted as a minus sign, thus the TCL error. But in the code the dash is replaced with an underscore so you shouldn't get the same error message with that code.

Furthermore, the class match is used with a class name but in this case you're using a variable containing a list of the elements in the class which isn't the same thing. Maybe you should try using the variable containing the class name instead in the class match statement:

[class match $cSSLSubject contains $dg_name]

I have never tried myself to use a variable instead of the actual object name so I don't know if that's an issue but I've seen people use the same concept for selecting pools so it might be okay.