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Regex.Replace Method (String, String, MatchEvaluator)

Updated: April 2009

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 static string Replace(
	string input,
	string pattern,
	MatchEvaluator evaluator
)

Parameters

input
Type: System.String

The string to search for a match.

pattern
Type: System.String

The regular expression pattern to match.

evaluator
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.

ExceptionCondition
ArgumentException

A regular expression parsing error has occurred.

ArgumentNullException

input is null.

-or-

pattern is null.

-or-

evaluator is null.

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 various 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 string MatchEvaluatorMethod(Match match)
{
   return String.Empty;
}

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<TKey, TValue>(TKey[], TValue[], IComparer<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:

\w+

Match one or more word characters.

using System;
using System.Collections;
using System.Text.RegularExpressions;

public class Example
{
   public static void Main()
   {
      string words = "letter alphabetical missing lack release " + 
                     "penchant slack acryllic laundry cease";
      string pattern = @"\w+";                            
      MatchEvaluator evaluator = new MatchEvaluator(WordScrambler);
      Console.WriteLine("Original words:");
      Console.WriteLine(words);
      Console.WriteLine();
      Console.WriteLine("Scrambled words:");
      Console.WriteLine(Regex.Replace(words, pattern, evaluator));      
   }

   public static string WordScrambler(Match match)
   {
      int arraySize = match.Value.Length;
      // Define two arrays equal to the number of letters in the match. 
      double[] keys = new double[arraySize];
      char[] letters = new char[arraySize];

      // Instantiate random number generator'
      Random rnd = new Random();

      for (int ctr = 0; ctr < match.Value.Length; ctr++)
      {
         // Populate the array of keys with random numbers.
         keys[ctr] = rnd.NextDouble();
         // Assign letter to array of letters.
         letters[ctr] = match.Value[ctr];
      }         
      Array.Sort(keys, letters, 0, arraySize, Comparer.Default);      
      return new String(letters);
   }
}
// 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

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

Date

History

Reason

April 2009

Expanded the Remarks section and added an example.

Customer feedback.

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