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Regular Expression Example: Changing Date Formats

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The following code example uses the Regex.Replace method to replace dates that have the form mm/dd/yy with dates that have the form dd-mm-yy.

Warning

When using System.Text.RegularExpressions to process untrusted input, pass a timeout. A malicious user can provide input to RegularExpressions, causing a Denial-of-Service attack. ASP.NET Core framework APIs that use RegularExpressions pass a timeout.

Example

static string MDYToDMY(string input)
{
   try {
      return Regex.Replace(input,
             @"\b(?<month>\d{1,2})/(?<day>\d{1,2})/(?<year>\d{2,4})\b",
            "${day}-${month}-${year}", RegexOptions.None,
            TimeSpan.FromMilliseconds(150));
   }
   catch (RegexMatchTimeoutException) {
      return input;
   }
}

Function MDYToDMY(input As String) As String
    Try
        Return Regex.Replace(input, _
               "\b(?<month>\d{1,2})/(?<day>\d{1,2})/(?<year>\d{2,4})\b", _
               "${day}-${month}-${year}", RegexOptions.None,
               TimeSpan.FromMilliseconds(150))
    Catch e As RegexMatchTimeoutException
        Return input
    End Try
End Function

The following code shows how the MDYToDMY method can be called in an application.

using System;
using System.Globalization;
using System.Text.RegularExpressions;

public class Class1
{
   public static void Main()
   {
      string dateString = DateTime.Today.ToString("d",
                                        DateTimeFormatInfo.InvariantInfo);
      string resultString = MDYToDMY(dateString);
      Console.WriteLine($"Converted {dateString} to {resultString}.");
   }

   static string MDYToDMY(string input)
   {
      try {
         return Regex.Replace(input,
                @"\b(?<month>\d{1,2})/(?<day>\d{1,2})/(?<year>\d{2,4})\b",
               "${day}-${month}-${year}", RegexOptions.None,
               TimeSpan.FromMilliseconds(150));
      }
      catch (RegexMatchTimeoutException) {
         return input;
      }
   }
}
// The example displays the following output to the console if run on 8/21/2007:
//      Converted 08/21/2007 to 21-08-2007.

Imports System.Globalization
Imports System.Text.RegularExpressions

Module DateFormatReplacement
    Public Sub Main()
        Dim dateString As String = Date.Today.ToString("d", _
                                             DateTimeFormatInfo.InvariantInfo)
        Dim resultString As String = MDYToDMY(dateString)
        Console.WriteLine("Converted {0} to {1}.", dateString, resultString)
    End Sub

    Function MDYToDMY(input As String) As String
        Try
            Return Regex.Replace(input, _
                   "\b(?<month>\d{1,2})/(?<day>\d{1,2})/(?<year>\d{2,4})\b", _
                   "${day}-${month}-${year}", RegexOptions.None,
                   TimeSpan.FromMilliseconds(150))
        Catch e As RegexMatchTimeoutException
            Return input
        End Try
    End Function
End Module
' The example displays the following output to the console if run on 8/21/2007:
'      Converted 08/21/2007 to 21-08-2007.

Comments

The regular expression pattern \b(?<month>\d{1,2})/(?<day>\d{1,2})/(?<year>\d{2,4})\b is interpreted as shown in the following table.

Pattern Description
\b Begin the match at a word boundary.
(?<month>\d{1,2}) Match one or two decimal digits. This is the month captured group.
/ Match the slash mark.
(?<day>\d{1,2}) Match one or two decimal digits. This is the day captured group.
/ Match the slash mark.
(?<year>\d{2,4}) Match from two to four decimal digits. This is the year captured group.
\b End the match at a word boundary.

The pattern ${day}-${month}-${year} defines the replacement string as shown in the following table.

Pattern Description
$(day) Add the string captured by the day capturing group.
- Add a hyphen.
$(month) Add the string captured by the month capturing group.
- Add a hyphen.
$(year) Add the string captured by the year capturing group.

See also