C＃如果正则表达式不匹配则执行某些操作

So I got piece of code, which works like if someone writes in chat !buy 3, then Regex gets that number 3 and does stuff, but how could I make code that if someone just types !buy, then it tells "him" something, else than throwing error to console because there were no number.So I got piece of code, which works like if som