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Regex.Replace Method (String, MatchEvaluator, Int32)

Updated: September 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
Dim instance As Regex 
Dim input As String 
Dim evaluator As MatchEvaluator 
Dim count As Integer 
Dim returnValue As String 

returnValue = instance.Replace(input, _
	evaluator, count)


Type: System.String

The string to search for a match.

Type: System.Text.RegularExpressions.MatchEvaluator

A custom method that examines each match and returns either the original matched string or a replacement string.

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.


input is Nothing.


evaluator is Nothing.

The Regex.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 Regex.Matches(String) method and passing the first count Match 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(match As Match) As String 
   Return String.Empty
End Function

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

Module Example
   Public Sub Main()
      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)
      Console.WriteLine("Original string: " + input)

      Dim result As String = rgx.Replace(input, AddressOf ReverseLetter, _ 
                                           input.Split(" "c).Length \ 2)
      Console.WriteLine("Returned string: " + result)
   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

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.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0




September 2010

Added an example.

Information enhancement.

April 2009

Expanded the Remarks section.

Customer feedback.