Regex.Replace Method (String, MatchEvaluator, Int32)

Updated: October 2010

Within a specified input string, replaces a specified maximum number of strings that match a regular expression pattern with a string returned by a MatchEvaluator delegate.

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


Public Function Replace ( _
    input As String, _
    evaluator As MatchEvaluator, _
    count As Integer _
) As String
public string Replace(
    string input,
    MatchEvaluator evaluator,
    int count


  • count
    Type: System.Int32
    The maximum number of times the replacement will occur.

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.


Exception Condition

input is nulla null reference (Nothing in Visual Basic).


evaluator is nulla null reference (Nothing in Visual Basic).


The Replace(String, MatchEvaluator, Int32) 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) method and passing the first countMatch objects in the returned MatchCollection collection to the evaluator delegate.

The regular expression is the pattern defined by the constructor for the current Regex object.

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
public string MatchEvaluatorMethod(Match match)

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


The following example uses a regular expression to deliberately misspell half of the words in a list. It uses the regular expression \w*(ie|ei)\w* to match words that include the characters "ie" or "ei". It passes the first half of the matching words to the ReverseLetter method, which, in turn, uses the Replace method to reverse "i" and "e" in the matched string. The remaining words remain unchanged.

Imports System.Text.RegularExpressions

Public Module Example
   Public Sub Demo(outputBlock As System.Windows.Controls.TextBlock)

      Dim input As String = "deceive relieve achieve belief fierce receive"
      Dim pattern As String = "\w*(ie|ei)\w*"
      Dim rgx As New Regex(pattern, RegexOptions.IgnoreCase)
      outputBlock.Text += "Original string: " + input + vbCrLf

      Dim result As String = rgx.Replace(input, AddressOf ReverseLetter, 
                                           input.Split(" "c).Length \ 2)
      outputBlock.Text += "Returned string: " + result + vbCrLf   
   End Sub

   Public Function ReverseLetter(match As Match) As String
      Return Regex.Replace(match.Value, "([ie])([ie])", "$2$1", 
   End Function
End Module
' The example displays the following output:
'    Original string: deceive relieve achieve belief fierce receive
'    Returned string: decieve releive acheive belief fierce receive
using System;
using System.Text.RegularExpressions;

public class Example
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
      string input = "deceive relieve achieve belief fierce receive";
      string pattern = @"\w*(ie|ei)\w*";
      Regex rgx = new Regex(pattern, RegexOptions.IgnoreCase);
      outputBlock.Text += "Original string: " + input + "\n";

      string result = rgx.Replace(input, new MatchEvaluator(Example.ReverseLetter),
                                  input.Split(' ').Length / 2);
      outputBlock.Text += "Returned string: " + result + "\n";

   static string ReverseLetter(Match match)
      return Regex.Replace(match.Value, "([ie])([ie])", "$2$1",
// The example displays the following output:
//    Original string: deceive relieve achieve belief fierce receive
//    Returned string: decieve releive acheive belief fierce receive

The regular expression \w*(ie|ei)\w* is defined as shown in the following table.




Match zero or more word characters.


Match either "ie" or "ei".


Match zero or more word characters.

The regular expression pattern ([ie])([ie]) in the ReverseLetter method matches the first "i" or "e" in the diphthong "ie" or "ei" and assigns the letter to the first capturing group. It matches the second "i" or "e" and assigns the letter to the second capturing group. The two characters are then reversed by calling the Replace method with the replacement pattern $2$1.

Version Information


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.

Change History




October 2010

Added an example.

Information enhancement.