Hold at Bay

 
Prerequisite(s): Base Attack Bonus 4+, Weapon Specialization with any weapon qualifying as a reach weapon
 
Benefit: When you are equipped with a "reach" weapon, an opponent who moves through your threat range, even if taking a five-foot-step, provokes an attack of opportunity. This attack of opportunity can only be made as a standard attack with the reach weapon, and it is resolved before the opponent completes his move.
 
Special: The opponent must begin more than 5 feet away--this feat does not allow you to use a reach weapon to attack an opponent already adjacent to you. This feat does not grant any special attack of opportunity against an opponent who moves out of your threat range (normal rules apply), nor does it allow an attack of opportunity for movement that normally negates attacks of opportunity (such as the Tumble skill). Hold At Bay may be taken as a fighter bonus feat.
 
Source: Net Book of Feats