As Abigail-II points out how is a computer supposed to do this? This is AI++. Take A-II's posts for example. Few here would doubt that A-II is indeed an expert, but the content delivered in a given A-II reply depends on all sorts of things from the quaility of the question through to the phases of the moon. Regardless of this fact very few users here have any problem recognising that A-II is indeed an expert.

Why?

Humans are really good at heuristic analysis, so use the force. Don't try to do it the hard way. Let your expert heuristic humans identify the subject experts for you with some sort of rating/XP type system. They will do it far more accurately than anything you are likely to knock up in the forseeable future.....You are trying to compete with computers far more complex than what we currently have readily available, each with 20+ years of training in dealing with the nuances of language.