when语句尽管可以给模式添加一些条件，但是当语句过于复杂的时候可以考虑某个分支的模式定义为一个方法：
open System.Text.RegularExpressions // create an active pattern to match an email address let (|EmailAddress|_|) input = let m = Regex.Match(input,@".+@.+") if (m.Success) then Some input else None // use the active pattern in the match let classifyString aString = match aString with | EmailAddress x -> printfn "%s is an email" x // otherwise leave alone | _ -> printfn "%s is something else" aString //test classifyString "alice@example.com" classifyString "google.com"