dynamic rex pattern

I'm trying to use rex to extract a value from an event.

In order to avoid writing out the pattern too many times, I have decided to place the pattern inside a macro with a specified argument passed in.

First, sample data looks like this:

DataType=1, PowerMax=50, PowerMin=10

To invoke the macro, I might use a command like this:

`macFoo("PowerMax")`

and inside the macro, I want to do something like this:

... | eval re=", ".$arg1$."=(?<bar>[^,]*)" | rex field=_raw re

which i wanted to extract bar=50, but I get an error similar to this:

Error in 'rex' command: The regex 're' does not extract anything. It should specify at least one named group. Format: (?<name>...).

Help!