Los Angeles: Singer Katy Perry says she will collaborate with Taylor Swift if the "I knew you were trouble" hitmaker apologises to her. Perry was answering to fans' questions on Twitter on Saturday when one user asked if she would ever collaborate with Swift, reports eonline.com. The two pop stars have been in a long-running feud that has yet to be settled, but the shade they have thrown at each other has never been a direct attack. "Katy Perry will you collaborate with Taylor swift," asked the user. "If she says sorry, sure!" Perry responded. The "Style" hitmaker and Perry's feud goes back to the time when Swift claimed in an interview that a pop star tried to steal her tour's dancers out from under her. Eventually people deduced that Swift was referring to Perry, who tweeted to fans: "Watch out for the Regina George in sheep's clothing."