Regex.Replace Method (String, String, Int32)


In a specified input string, replaces a specified maximum number of strings that match a regular expression pattern with a specified replacement string.

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

public string Replace(
	string input,
	string replacement,
	int count


Type: System.String

The string to search for a match.

Type: System.String

The replacement string.

Type: System.Int32

The maximum number of times the replacement can occur.

Return Value

Type: System.String

A new string that is identical to the input string, except that the replacement string takes the place of each matched string. If the regular expression pattern is not matched in the current instance, the method returns the current instance unchanged.

Exception Condition

input or replacement is null.


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

The search for matches starts at the beginning of the input string. The regular expression is the pattern that is defined by the constructor for the current Regex object. If count is negative, replacements continue to the end of the string. If count exceeds the number of matches, all matches are replaced.

The replacement parameter specifies the string that is to replace the first count matches in input. replacement can consist of any combination of literal text and substitutions. For example, the replacement pattern a*${test}b inserts the string "a*" followed by the substring that is matched by the test capturing group, if any, followed by the string "b". The * character is not recognized as a metacharacter within a replacement pattern.


Substitutions are the only regular expression language elements that are recognized in a replacement pattern. All other regular expression language elements, including character escapes, are allowed in regular expression patterns only and are not recognized in replacement patterns.

The RegexMatchTimeoutException exception is thrown if the execution time of the replacement operation exceeds the time-out interval specified by the Regex.Regex(String, RegexOptions, TimeSpan) constructor. If you do not set a time-out interval when you call the constructor, the exception is thrown if the operation exceeds any time-out value established for the application domain in which the Regex object is created. If no time-out is defined in the Regex constructor call or 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.

The following example replaces the first five occurrences of duplicated characters with a single character. The regular expression pattern (\w)\1 matches consecutive occurrences of a single character and assigns the first occurrence to the first capturing group. The replacement pattern $1 replaces the entire match with the first captured group.

using System;
using System.Text.RegularExpressions;

public class Example
   public static void Main()
      string str = "aabccdeefgghiijkklmm";
      string pattern = "(\\w)\\1"; 
      string replacement = "$1"; 
      Regex rgx = new Regex(pattern);

      string result = rgx.Replace(str, replacement, 5);
      Console.WriteLine("Original String:    '{0}'", str);
      Console.WriteLine("Replacement String: '{0}'", result); 
// The example displays the following output:
//       Original String:    'aabccdeefgghiijkklmm'
//       Replacement String: 'abcdefghijkklmm'

Universal Windows Platform
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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