Lambda Expression in Powershell

Question

I have a code in C# which uses lambda expressions for delegate passing to a method. How can I achieve this in PowerShell. For example the following is a C# code:

string input = "(,)(;)(:)(!)";
string pattern = @"\((?<val>[\,\!\;\:])\)";
var r = new Regex(pattern);
string result = r.Replace(input, m =>
    {
        if (m.Groups["val"].Value == ";") return "[1]";
        else return "[0]";
    });
Console.WriteLine(result);

And this is the PowerShell script without the lambda-expression in place:

$input = "(,)(;)(:)(!)";
$pattern = "\((?<val>[\,\!\;\:])\)";
$r = New-Object System.Text.RegularExpressions.Regex $pattern
$result = $r.Replace($input, "WHAT HERE?")
Write-Host $result

Note: my question is not about solving this regular-expression problem. I just want to know how to pass a lambda expression to a method that receives delegates in PowerShell.

Stefan · Accepted Answer · 2012-06-12 11:41:35Z

79

In PowerShell 2.0 you can use a script block ({ some code here }) as delegate:

$MatchEvaluator = 
{  
  param($m) 

  if ($m.Groups["val"].Value -eq ";") 
  { 
    #... 
  }
}

$result = $r.Replace($input, $MatchEvaluator)

Or directly in the method call:

$result = $r.Replace($input, { param ($m) bla })

Tip:

You can use [regex] to convert a string to a regular expression:

$r = [regex]"\((?<val>[\,\!\;\:])\)"
$r.Matches(...)

edited Jun 12, 2012 at 11:41

answered Jun 12, 2012 at 11:36

Stefan

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2 Comments

Sina Iravanian Over a year ago

And thanks also for mentioning the param($m) syntax inside the script-blocks.

Joe Phillips Over a year ago

If anyone would like a real example: $repo.Deployments.FindOne({ param($d) $d.EnvironmentId -eq $envId });

Randall Bohn · Accepted Answer · 2013-06-21 21:53:45Z

33

Sometimes you just want something like this:

{$args[0]*2}.invoke(21)

(which will declare an anonymous 'function' and call it immediately.)

answered Jun 21, 2013 at 21:53

Randall Bohn

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Comments

Shay Levy · Accepted Answer · 2012-06-12 11:49:23Z

18

You can use this overload

[regex]::replace(
   string input,
   string pattern, 
   System.Text.RegularExpressions.MatchEvaluator evaluator
)

The delegate is passes as a scriptblock (lambda expression) and the MatchEvaluator can be accessed via the $args variable

[regex]::replace('hello world','hello', { $args[0].Value.ToUpper() })

answered Jun 12, 2012 at 11:49

Shay Levy

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4 Comments

user189198 Over a year ago

Can you use a ScriptBlock in place of any delegate? Where is this documented?

Shay Levy Over a year ago

Generally speaking, yes. I'm not aware of any documentation.

Sina Iravanian Over a year ago

Thanks this works. And +1 for mentioning the $args array inside the script block.

Shay Levy Over a year ago

Thanks! Alternatively you can define named parameters with the param keyword.

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