Regular Expression Example: Changing Date Formats

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

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 {0} to {1}.", dateString, 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

.NET Regular Expressions