f# – 如何避免两次写正则表达式匹配？

发布时间：2020-12-14 05:56:24 所属栏目：百科来源：网络整理

导读：我有这个代码： let main argv = let x="b"match x with| _ when Regex.Match(x,"a").Success=true- let a=Regex.Match(x,"a") Console.WriteLine(a.Groups.[0])| _ when Regex.Match(x,"b").Success=true- let b=Regex.Match(x,"b") Console.WriteLine(b.Gr

我有这个代码：

let main argv = 
let x="b"
match x with
| _ when Regex.Match(x,"a").Success=true->
    let a=Regex.Match(x,"a")
    Console.WriteLine(a.Groups.[0])
| _ when Regex.Match(x,"b").Success=true->
    let b=Regex.Match(x,"b")
    Console.WriteLine(b.Groups.[0])

我必须在上面的代码中两次写Regex.Match(x,“a”)和Regex.Match(x,“b”).有没有办法只写一次？

解决方法

寻找活跃的模式：

let (|Match|_|) pattern input =
    let m = Regex.Match(input,pattern,RegexOptions.Singleline)
    if m.Success 
        then Some (m.Groups.[0])
        else None
// and use it:
match x with
| Match @"a text" a ->
    printfn "found a: %s" a
| Match @"b text" b ->
    printfn "found b: %s" b

您可以更进一步,创建一个支持其自身解析值的活动模式：

let (|Regex|_|) pattern input =
    let m = Regex.Match(input,RegexOptions.Singleline)
    if m.Success
        // List.tail is needed to skip the m.Groups.[0] that returns the entire parsed scope
        then [ for g in m.Groups -> g.Value ] |> List.tail |> Some
        else None

match x with
| Regex @"a (w+)s(w+)" [ a1; a2 ] ->
    printfn "found A value: %s and %s" a1 a2
| Regex @"b (w+)s(w+)s(w+)" [ b1; b2; b3 ] ->
    printfn "found B value: %s and %s and %s" b1 b2 b3

// this will be able to parse the following:
// "a foo bar"     ==> a1="foo" and a2="bar"
// "b foo bar baz" ==> b1="foo",b2="bar",and b3="baz"

（编辑：李大同）

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