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

Within 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 a replacement string takes the place of each matched string.


input or replacement is null.


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

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 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.

.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.