Write RegExp-Vars in a function

Hi, Although this seemed to be an easy prob I couldn't solve it in all the time. I have a function and a string i want to be replaced. I use preg_replace and get a var out. Now I want this var in the function but I don't know how I can get it in: preg_replace("#\[\+\+(.*)\+\+\]#Uis",my_function($1),$str); doesnt work at all and lots of other variations do not as well. Please to be helping. Greets Niels