How to iterate over DataGroup List elements ?

I didn't write the logic behind the data groups but it seems that it is internally represented by a string. By adding the TCL split command to create a true TCL list, foreach should work. I'll verify if this is something we need to fix or if it's by design.

...  
   foreach i [split $::my_str_class] {  
     log local0. $i  
   }  
   ...

As for examples, we're working on that. As for the usage of matchclass, do a search in the forums and you'll find plenty of examples. There is also an example in chapter 13 (Writing iRules) of the BIG-IP v9 user reference.

-Joe