Regex Powertoy had a bug where capturing groups inside unmatched alternations would prevent group details from displaying properly, when clicking on a match.
It's fixed now, but this matchmark would reproduce the problem -- clicking on the single match would show no group details. Now, the details show -- and any groups unmatched aren't shown at all in the detail view.
Regex Powertoy more or less works as long as you've got a modern browser and Java applet support at the "5.0" (aka "JDK 1.5") level. Any bug/glitch reports are appreciated -- and if they're specific to a certain regex pattern, target input, or other settings, you can use the 'matchmark' function to convert your settings to an URL for easy reporting.