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Regex.Replace Method (String, String)

In a specified input string, replaces all 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
)

Parameters

input
Type: System.String

The string to search for a match.

replacement
Type: System.String

The replacement string.

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.

ExceptionCondition
ArgumentNullException

input or replacement is null.

RegexMatchTimeoutException

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 defined by the constructor for the current Regex object.

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.

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 defines a regular expression, \s+, that matches one or more white-space characters. The replacement string, " ", replaces them with a single space character.

using System;
using System.Text.RegularExpressions;

public class Example
{
   public static void Main()
   {
      string input = "This is   text with   far  too   much   " + 
                     "whitespace.";
      string pattern = "\\s+";
      string replacement = " ";
      Regex rgx = new Regex(pattern);
      string result = rgx.Replace(input, replacement);

      Console.WriteLine("Original String: {0}", input);
      Console.WriteLine("Replacement String: {0}", result);                             
   }
}
// The example displays the following output: 
//       Original String: This is   text with   far  too   much   whitespace. 
//       Replacement String: This is text with far too much whitespace.

The following example defines a regular expression, (\p{Sc}\s?)?(\d+\.?((?<=\.)\d+)?)(?(1)|\s?\p{Sc})?, and a replacement pattern, $2, that removes either a leading or a trailing currency symbol from a numeric value.

using System;
using System.Text.RegularExpressions;

public class Example
{
   public static void Main()
   {
      string pattern =  @"(\p{Sc}\s?)?(\d+\.?((?<=\.)\d+)?)(?(1)|\s?\p{Sc})?";
      string input = "$17.43  €2 16.33  £0.98  0.43   £43   12€  17";
      string replacement = "$2";
      Regex rgx = new Regex(pattern);
      string result = rgx.Replace(input, replacement);

      Console.WriteLine("Original String:    '{0}'", input);
      Console.WriteLine("Replacement String: '{0}'", result);                             
   }
}
// The example displays the following output: 
//       Original String:    '$17.43  €2 16.33  £0.98  0.43   £43   12€  17' 
//       Replacement String: '17.43  2 16.33  0.98  0.43   43   12  17'

The regular expression is interpreted as shown in the following table.

Pattern

Description

\p{Sc}

Match a currency symbol. {Sc} denotes any character that is a member of the Unicode Symbol, Currency category.

\s?

Match zero or one white-space character.

(\p{Sc}\s?)?

Match zero or one occurrence of the combination of a currency symbol followed by zero or one white-space character. This is the first capturing group.

\d+

Match one or more decimal digits.

\.?

Match zero or one occurrence of a period (used as a decimal separator character).

((?<=\.)\d+)?

If a period is the previous character, match one or more decimal digits. This pattern can be matched either zero or one time.

(\d+\.?((?<=\.)\d+)?)

Match the pattern of one or more decimal digits followed by an optional period and additional decimal digits. This is the second capturing group. The call to the Replace(String, String) method replaces the entire match with the value of this captured group.

(?(1)|\s?\p{Sc})?

If the first captured group exists, match an empty string. Otherwise, match zero or one white-space character followed by a currency symbol.

.NET Framework

Supported in: 4.5, 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

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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