Oh, I misunderstood. I thought you meant a literal “get” instead of actually setting rich text. This has got to be easier with RegEx, which Axure doesn’t support, but you should be able to do with js injection. Here is a thread for basically injecting regex support into Axure:
Now, I’ll bet javascript calls won’t work in a repeater, as you’ll need some delay–at least 50 ms–between js calls, which you can’t do or rely on in an Item Loaded event. …But that’s just a guess. You could try calling a “listener event” row by row maybe, and set the text via js, wait a bit then process the next row.
Doing this in pure Axure could be a pain, but seems like it could maybe work if you can set up a recursive loop to set the rich text char by char. Maybe don’t use Item Loaded to set the text but instead use a “listener event” like Move and use that to trigger a function to set rich text. My idea here is a recursive loop (like change dp state every 100 ms) that tests the first char only and if an ‘a’ it calls Set Text (rich text) setting that char to red, if not set that char to black; then (loop and) test the second, etc.