Arsenal manager Arsene Wenger has warned Manchester United that they may look back on the Champions League semi-final first leg at Old Trafford and wished they had scored more than one goal.While the Frenchman admits it would be the Gunners' greatest ever achievement against the Red Devils, he is confident his side will punish the holders in the return clash next Tuesday for not making the most of their chances."They can have regrets because they did not score a second goal," Wenger is quoted as saying by The Sun."It is down to us to make sure they regret they didn’t."The return leg will be a good opportunity to show our character and mental strength."I am confident that you will see our strength of character — and next Tuesday I can talk about it in the press conference after the game."It will be our biggest win over United if we do it because there is a final at stake."And when we play at home and know that there is a final at stake we will be up for it."We will be up for it next Tuesday and I am confident we can do it. I believe we will be on top."Source: Goal.com