Yahoo's anti-abuse AI can hunt out even the most devious online trolls

But keyword-based systems – similar to a spam filters – aren't great at catching more subtle types of hate speech. Trolls might deliberately obscure abusive words so they sneak past filters or posts might be hateful without ever containing a particular abusive keyword. Existing algorithms can also mislabel sarcastic comments as abusive, fail to recognise new ways of expressing hate, or ignore a troll's post because it was written in grammatically correct English.