• Article

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

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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)

Syntax

'Declaration
Public Function Replace ( _
    input As String, _
    replacement As String, _
    count As Integer, _
    startat As Integer _
) As String

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

Parameters

  • count
    Type: System.Int32
    Maximum number of times the replacement can occur.
  • startat
    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.

Exceptions

Exception Condition
ArgumentNullException

input is nulla null reference (Nothing in Visual Basic).

-or-

replacement is nulla null reference (Nothing in Visual Basic).
ArgumentOutOfRangeException

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

Remarks

The search for matches starts in the input parameter 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.

NoteNote:

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.

Examples

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.

Imports System.Text.RegularExpressions

Module Example
   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      Dim input As String = "Instantiating a New Type" + vbCrLf + _
                            "Generally, there are two ways that an" + vbCrLf + _
                            "instance of a class or structure can" + vbCrLf + _
                            "be instantiated. "
      Dim pattern As String = "^.*$"
      Dim replacement As String = vbCrLf + "$&"
      Dim rgx As New Regex(pattern, RegexOptions.Multiline)
      Dim result As String = String.Empty

      Dim match As Match = rgx.Match(input)
      ' Double space all but the first line.
      If match.Success Then
         result = rgx.Replace(input, replacement, -1, match.Index + match.Length + 1)
      End If
      outputBlock.Text &= result & vbCrLf
   End Sub
End Module
' 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.

using System;
using System.Text.RegularExpressions;

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      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);

      outputBlock.Text += result + "\n";
   }
}
// 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.

Pattern

Description

^

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.

Version Information

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

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.