Regex.Replace Method (String, String, MatchEvaluator)

Within a specified input string, replaces all strings that match a specified regular expression with a string returned by a MatchEvaluator delegate.

Namespace:  System.Text.RegularExpressions
Assembly:  System (in System.dll)

Public Shared Function Replace ( _
	input As String, _
	pattern As String, _
	evaluator As MatchEvaluator _
) As String


Type: System.String
The string to search for a match.
Type: System.String
The regular expression pattern to match.
Type: System.Text.RegularExpressions.MatchEvaluator
A custom method that examines each match and returns either the original matched string or a replacement string.

Return Value

Type: System.String
A new string that is identical to the input string, except that a replacement string takes the place of each matched string.


A regular expression parsing error occurred.


input is Nothing.


pattern is Nothing.


evaluator is Nothing.

The Replace(String, String, MatchEvaluator) method is useful for replacing a regular expression match if any of the following conditions is true:

  • The replacement string cannot readily be specified by a regular expression replacement pattern.

  • The replacement string results from some processing done on the matched string.

  • The replacement string results from conditional processing.

The method is equivalent to calling the Matches(String, String) method and passing each Match object in the returned MatchCollection collection to the evaluator delegate.

The pattern parameter consists of various regular expression language elements that symbolically describe the string to match. For more information about regular expressions, see Regular Expression Language Elements in the .NET Framework documentation.

The evaluator parameter is the delegate for a custom method that you define and that examines each match. The custom method must have the following signature to match the MatchEvaluator delegate.

Public Function MatchEvaluatorMethod(ByVal match As Match) As String

Your custom method returns a string that replaces the matched input.

The following example uses a regular expression to extract the individual words from a string and then uses a MatchEvaluator delegate to call a method named WordScramble that scrambles the individual letters in the word. To do this, the WordScramble method creates an array that contains the characters in the match. It also creates a parallel array that it populates with random floating-point numbers. The arrays are sorted by calling the Array.Sort(Of TKey, TValue)(TKey(), TValue(), IComparer(Of TKey)) method, and the sorted array is provided as an argument to a String class constructor. This newly created string is then returned by the WordScramble method. The regular expression is interpreted as follows:


Match one or more word characters.

Imports System.Collections.Generic
Imports System.Text.RegularExpressions

Module Example
   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      Dim words As String = "letter alphabetical missing lack release " + _
                            "penchant slack acryllic laundry cease"
      Dim pattern As String = "\w+"
      Dim evaluator As MatchEvaluator = AddressOf WordScrambler
      outputBlock.Text &= "Original words:" & vbCrLf
      outputBlock.Text &= words & vbCrLf
      outputBlock.Text &= "Scrambled words:" & vbCrLf
      outputBlock.Text &= Regex.Replace(words, pattern, evaluator) & vbCrLf
   End Sub

   Public Function WordScrambler(ByVal match As Match) As String
      Dim arraySize As Integer = match.Value.Length - 1
      ' Define two arrays equal to the number of letters in the match.
      Dim keys(arraySize) As Double
      Dim letters(arraySize) As Char

      ' Instantiate random number generator'
      Dim rnd As New Random()

      For ctr As Integer = 0 To match.Value.Length - 1
         ' Populate the array of keys with random numbers.
         keys(ctr) = rnd.NextDouble()
         ' Assign letter to array of letters.
         letters(ctr) = match.Value.Chars(ctr)
      Array.Sort(keys, letters, 0, arraySize, Comparer(Of Double).Default)
      Return New String(letters)
   End Function
End Module
' The example displays output similar to the following:
'    Original words:
'    letter alphabetical missing lack release penchant slack acryllic laundry cease
'    Scrambled words:
'    elrtte iaeabatlpchl igmnssi lcka aerslee hnpatnce ksacl lialcryc dylruna ecase


Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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