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Regex.Replace Method (String, MatchEvaluator, Int32)

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 string Replace(
	string input,
	MatchEvaluator evaluator,
	int 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 or evaluator is null.

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 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.

using System;
using System.Text.RegularExpressions;

public class Example
   public static void Main()
      string input = "deceive relieve achieve belief fierce receive";
      string pattern = @"\w*(ie|ei)\w*";
      Regex rgx = new Regex(pattern, RegexOptions.IgnoreCase);
      Console.WriteLine("Original string: " + input);

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

   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.

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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