Using Active patterns

Show's how to create a parametrized active pattern that will match based on a regular expression.

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open System.Text.RegularExpressions

// the definition of the active pattern
let (|Regex|_|) regexPattern input =
    // create and attempt match regular expression
    let regex = new Regex(regexPattern)
    let regexMatch = regex.Match(input)
    // return either Some or None
    if regexMatch.Success then
        Some regexMatch.Value
    else
        None

// function to print the results by pattern
// matching over different instances of the
// active pattern
let printInputWithType input =
    match input with
    | Regex "$true|false^" s -> printfn "Boolean: %s" s
    | Regex @"$-?\d+^" s -> printfn "Integer: %s" s
    | Regex "$-?\d+\.\d*^" s -> printfn "Floating point: %s" s
    | _ -> printfn "String: %s" input

// print the results    
printInputWithType "true"
printInputWithType "12"
printInputWithType "-12.1"

namespace System

namespace System.Text

namespace System.Text.RegularExpressions

Multiple items
type Regex =
  new : pattern:string -> Regex + 1 overload
  member GetGroupNames : unit -> string[]
  member GetGroupNumbers : unit -> int[]
  member GroupNameFromNumber : i:int -> string
  member GroupNumberFromName : name:string -> int
  member IsMatch : input:string -> bool + 1 overload
  member Match : input:string -> Match + 2 overloads
  member Matches : input:string -> MatchCollection + 1 overload
  member Options : RegexOptions
  member Replace : input:string * replacement:string -> string + 5 overloads
  ...

Full name: System.Text.RegularExpressions.Regex

--------------------
Regex(pattern: string) : unit
Regex(pattern: string, options: RegexOptions) : unit

val regexPattern : string

val input : string

val regex : Regex

val regexMatch : Match

Regex.Match(input: string) : Match
Regex.Match(input: string, startat: int) : Match
Regex.Match(input: string, beginning: int, length: int) : Match

property Group.Success: bool

union case Option.Some: Value: 'T -> Option<'T>

property Capture.Value: string

union case Option.None: Option<'T>

val printInputWithType : input:string -> unit

Full name: Script.printInputWithType

Multiple items
active recognizer Regex: string -> string -> string option

Full name: Script.( |Regex|_| )

--------------------
type Regex =
  new : pattern:string -> Regex + 1 overload
  member GetGroupNames : unit -> string[]
  member GetGroupNumbers : unit -> int[]
  member GroupNameFromNumber : i:int -> string
  member GroupNumberFromName : name:string -> int
  member IsMatch : input:string -> bool + 1 overload
  member Match : input:string -> Match + 2 overloads
  member Matches : input:string -> MatchCollection + 1 overload
  member Options : RegexOptions
  member Replace : input:string * replacement:string -> string + 5 overloads
  ...

Full name: System.Text.RegularExpressions.Regex

--------------------
Regex(pattern: string) : unit
Regex(pattern: string, options: RegexOptions) : unit

val s : string

val printfn : format:Printf.TextWriterFormat<'T> -> 'T

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.printfn

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More information

Link:	http://fssnip.net/C
Posted:	14 years ago
Author:	Robert Pickering
Tags:	active pattern