'ao is indeed a problem because the ' means the word break has already happenened. But what's the problem with \bko\b or \ban\b? Or did you mean it exactly as you typed it: "ko" or "an" followed by a comma instead of a space? If they never actually occur in this context you can ignore the problem. Otherwise you have to be more exact. On the one side:
And on the other
That's assuming you're allowed to use non-capturing lookaheads. It works in my text editor-- that is, it picks up the intended forms without the unintended ones, and it's OK with the \b where I put it. But that's a different RegEx dialect (I've got a choice of eight "flavors" but default to Ruby) so I can't swear it would work universally.