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Regex.Replace Method (String, String, MatchEvaluator, RegexOptions)

Updated: September 2010

Within a specified input string, replaces all strings that match a specified regular expression with a string returned by a MatchEvaluator delegate. Specified options modify the matching operation.

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

'Declaration
Public Shared Function Replace ( _
	input As String, _
	pattern As String, _
	evaluator As MatchEvaluator, _
	options As RegexOptions _
) As String
'Usage
Dim input As String 
Dim pattern As String 
Dim evaluator As MatchEvaluator 
Dim options As RegexOptions 
Dim returnValue As String 

returnValue = Regex.Replace(input, _
	pattern, evaluator, options)

Parameters

input
Type: System.String

The string to search for a match.

pattern
Type: System.String

The regular expression pattern to match.

evaluator
Type: System.Text.RegularExpressions.MatchEvaluator

A custom method that examines each match and returns either the original matched string or a replacement string.

options
Type: System.Text.RegularExpressions.RegexOptions

A bitwise OR combination of the enumeration values.

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.

ExceptionCondition
ArgumentException

A regular expression parsing error has occurred.

ArgumentNullException

input is Nothing.

-or-

pattern is Nothing.

-or-

evaluator is Nothing.

ArgumentOutOfRangeException

options is not a valid bitwise combination of RegexOptions values.

The Regex.Replace(String, String, MatchEvaluator, RegexOptions) method is useful for replacing a regular expression match in if any of the following conditions is true:

  • The replacement string cannot readily be specified by a regular expression replacement pattern.

  • The replacement string results from some processing done on the matched string.

  • The replacement string results from conditional processing.

The method is equivalent to calling the Regex.Matches(String, String, RegexOptions) method and passing each Match object in the returned MatchCollection collection to the evaluator delegate.

The pattern parameter consists of various regular expression language elements that symbolically describe the string to match. For more information about regular expressions, see .NET Framework Regular Expressions and Regular Expression Language - Quick Reference.

The evaluator parameter is the delegate for a custom method that you define and that examines each match. The custom method must have the following signature to match the MatchEvaluator delegate.

Public Function MatchEvaluatorMethod(match As Match) As String 
   Return String.Empty
End Function

Your custom method returns a string that replaces the matched input.

If the options parameter specifies the RightToLeft enumeration value, the search for matches begins from the end of the input string and proceeds from right to left; otherwise, the search begins from the start of the input string and proceeds from left to right.

The following example uses a regular expression to extract the individual words from a string, and then uses a MatchEvaluator delegate to call a method named WordScramble that scrambles the individual letters in the word. To do this, the WordScramble method creates an array that contains the characters in the match. It also creates a parallel array that it populates with random floating-point numbers. The arrays are sorted by calling the Array.Sort(Of TKey, TValue)(TKey(), TValue(), IComparer(Of TKey)) method, and the sorted array is provided as an argument to a String class constructor. This newly created string is then returned by the WordScramble method. The regular expression pattern \w+ matches one or more word characters; the regular expression engine will continue to add characters to the match until it encounters a non-word character, such as a white-space character. The call to the Replace method includes the RegexOptions.IgnorePatternWhitespace option so that the comment in the regular expression pattern \w+ # Matches all the characters in a word. is ignored by the regular expression engine.

Imports System.Collections
Imports System.Text.RegularExpressions

Module Example
   Public Sub Main()
      Dim words As String = "letter alphabetical missing lack release " + _
                            "penchant slack acryllic laundry cease" 
      Dim pattern As String = "\w+  # Matches all the characters in a word."                             
      Dim evaluator As MatchEvaluator = AddressOf WordScrambler
      Console.WriteLine("Original words:")
      Console.WriteLine(words)
      Console.WriteLine("Scrambled words:")
      Console.WriteLine(Regex.Replace(words, pattern, evaluator, _
                                      RegexOptions.IgnorePatternWhitespace))      
   End Sub 

   Public Function WordScrambler(match As Match) As String 
      Dim arraySize As Integer = match.Value.Length - 1
      ' Define two arrays equal to the number of letters in the match. 
      Dim keys(arraySize) As Double 
      Dim letters(arraySize) As Char 

      ' Instantiate random number generator' 
      Dim rnd As New Random()

      For ctr As Integer = 0 To match.Value.Length - 1
         ' Populate the array of keys with random numbers.
         keys(ctr) = rnd.NextDouble()
         ' Assign letter to array of letters.
         letters(ctr) = match.Value.Chars(ctr)
      Next         
      Array.Sort(keys, letters, 0, arraySize, Comparer.Default)      
      Return New String(letters)
   End Function 
End Module 
' The example displays output similar to the following: 
'    Original words: 
'    letter alphabetical missing lack release penchant slack acryllic laundry cease 
'     
'    Scrambled words: 
'    etlert liahepalbcat imsgsni alkc ereelsa epcnnaht lscak cayirllc alnyurd ecsae

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

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

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

Date

History

Reason

September 2010

Added an example.

Customer feedback.

July 2009

Included additional exception information.

Content bug fix.

April 2009

Expanded the Remarks section.

Customer feedback.

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