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Regex.Replace Method (String, String, MatchEvaluator, RegexOptions)

Updated: October 2010

Within a specified input string, replaces all strings that match a specified regular expression with a string returned by a MatchEvaluator delegate. Specified options modify the matching operation.

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

public static string Replace(
	string input,
	string pattern,
	MatchEvaluator evaluator,
	RegexOptions options
)

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.
options
Type: System.Text.RegularExpressions.RegexOptions
A bitwise combination of the enumeration values.

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 occurred.

ArgumentNullException

input is null.

-or-

pattern is null.

-or-

evaluator is null.

ArgumentOutOfRangeException

options is not a valid bitwise combination of RegexOptions values.

The Replace(String, String, MatchEvaluator, RegexOptions) method is useful for replacing a regular expression match in 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, String, RegexOptions) 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 Regular Expression Language Elements in the .NET Framework documentation.

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.

If the options parameter specifies the RightToLeft enumeration value, the search for matches begins from the end of the input string and proceeds from right to left; otherwise, the search begins from the start of the input string and proceeds from left to right.

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 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. The call to the Replace method includes the RegexOptions.IgnorePatternWhitespace option so that the comment in the regular expression pattern \w+ # Matches all the characters in a word. is ignored by the regular expression engine.


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

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      string words = "letter alphabetical missing lack release " +
                     "penchant slack acryllic laundry cease";
      string pattern = @"\w+  # Matches all the characters in a word.";
      MatchEvaluator evaluator = new MatchEvaluator(WordScrambler);
      outputBlock.Text += "Original words:" + "\n";
      outputBlock.Text += words + "\n";
      outputBlock.Text += "\n";
      outputBlock.Text += "Scrambled words:" + "\n";
      outputBlock.Text += Regex.Replace(words, pattern, evaluator,
                                      RegexOptions.IgnorePatternWhitespace) + "\n";
   }

   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, Comparer<Double>.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:
//    etlert liahepalbcat imsgsni alkc ereelsa epcnnaht lscak cayirllc alnyurd ecsae


Silverlight

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.

Date

History

Reason

October 2010

Added an example.

Customer feedback.

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