Or, just keep it simple and make $regsub() and $regsubex() take the same parameters -- the only difference being that the EXtended version (aka $regsubEx) will evaluate the subtext, whereas $regsub does not evaluate the subtext. We might even make the %variable parameter in $regsub() optional and also return OutputText instead of N when %var is omitted.
That way it's easy and straight forward to explain in the help file, for new users to understand and grasp. I'll even help author the section of /help to make it nice.
