Regex.Replace Method (String, String, MatchEvaluator)

In 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
Assemblies:   System.Text.RegularExpressions (in System.Text.RegularExpressions.dll)
  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. If pattern is not matched in the current instance, the method returns the current instance unchanged.


A regular expression parsing error occurred.


input, pattern, or evaluator is Nothing.


A time-out occurred. For more information about time-outs, see the Remarks section.

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

The pattern parameter consists of regular expression language elements that symbolically describe the string to match. For more information about regular expressions, see .NET Framework Regular Expressions and Regular Expression Language - Quick Reference.

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

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

The RegexMatchTimeoutException exception is thrown if the execution time of the replacement operation exceeds the time-out interval specified for the application domain in which the method is called. If no time-out is defined in the application domain's properties, or if the time-out value is Regex.InfiniteMatchTimeout, no exception is thrown.

Because the method returns input unchanged if there is no match, you can use the Object.ReferenceEquals method to determine whether the method has made any replacements to the input string.

Notes to Callers

This method times out after an interval that is equal to the default time-out value of the application domain in which it is called. If a time-out value has not been defined for the application domain, the value Regex.InfiniteMatchTimeout, which prevents the method from timing out, is used. The recommended static method for evaluating and replacing a pattern match is Regex.Replace(String, String, MatchEvaluator, RegexOptions, TimeSpan), which lets you set the time-out interval.

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 pattern \w+ matches one or more word characters; the regular expression engine will continue to add characters to the match until it encounters a non-word character, such as a white-space character.

Imports System.Collections
Imports System.Text.RegularExpressions

Module Example
   Public Sub Main()
      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
      Console.WriteLine("Original words:")
      Console.WriteLine("Scrambled words:")
      Console.WriteLine(Regex.Replace(words, pattern, evaluator))      
   End Sub 

   Public Function WordScrambler(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.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

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0, 2.0, 1.0

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Portable Class Library

Supported in: Portable Class Library
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