Regex.Replace Method (String, String, Int32, Int32)


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In a specified input substring, 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,
	int startat


Type: System.String

The string to search for a match.

Type: System.String

The replacement string.

Type: System.Int32

Maximum number of times the replacement can occur.

Type: System.Int32

The character position in the input string where the search begins.

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.


startat is less than zero or greater than the length of input.


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

The search for matches starts in the input string at the position specified by the startat parameter. The regular expression is the pattern 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 each match 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 double-spaces all but the first line of a string. It defines a regular expression pattern, ^.*$, that matches a line of text, calls the Match(String) method to match the first line of the string, and uses the Match.Index and Match.Count properties to determine the starting position of the second line.

using System;
using System.Text.RegularExpressions;

public class Example
   public static void Main()
      string input = "Instantiating a New Type\n" +
                     "Generally, there are two ways that an\n" + 
                     "instance of a class or structure can\n" +
                     "be instantiated. ";
      string pattern = "^.*$";
      string replacement = "\n$&";
      Regex rgx = new Regex(pattern, RegexOptions.Multiline);
      string result = String.Empty; 

      Match match = rgx.Match(input);
      // Double space all but the first line.
      if (match.Success) 
         result = rgx.Replace(input, replacement, -1, match.Index + match.Length + 1);

// The example displays the following output:
//       Instantiating a New Type
//       Generally, there are two ways that an
//       instance of a class or structure can
//       be instntiated.

The regular expression pattern ^.*$ is defined as shown in the following table.




Match the start of a line. (Note that the Regex object was instantiated by using the RegexOptions.Multiline option; otherwise, this character class would only match the beginning of the input string.)


Match any character zero or more times.


Match the end of a line. (Note that the Regex object was instantiated by using the RegexOptions.Multiline option; otherwise, this character class would only match the beginning of the input string.)

The replacement string (vbCrLf + "$&" in Visual Basic, "\n$&" in C#) adds a new line before the matched string. Note that \n in the C# example is interpreted as the newline character by the C# compiler; it does not represent a regular expression character escape.

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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