Anelka left the Gunners for Real Madrid in a bitter £22million transfer in 1999, having arrived in London from Paris St Germain for just £500,000 two years earlier.
Arsenal boss Wenger said: "When he moved to Real Madrid, it was a regret. Yes, it was my biggest regret. He was in the French team then he moved to Madrid, Thierry Henry moved here and Thierry played in the European Championships and Nicolas did not play.
"At that time, it was a big mistake. Now, I still think Nicolas loves Arsenal. I was interested in Nicolas in the summer but not now.
"When Henry wanted to leave, I considered whether we were strong enough but we went with Bendtner and then we bought Eduardo.
"I am glad he has gone back to a top club."