Regex.Replace
using System; using System.Text; using System.Text.RegularExpressions; public class MainClass { static void Main( string[] args ) { // Create regex to search for IP address pattern. string pattern = @"(?<part1>[01]?\d\d?|2[0-4]\d|25[0-5])\." + @"(?<part2>[01]?\d\d?|2[0-4]\d|25[0-5])\." + @"(?<part3>[01]?\d\d?|2[0-4]\d|25[0-5])\." + @"(?<part4>[01]?\d\d?|2[0-4]\d|25[0-5])"; Regex regex = new Regex( pattern ); Match match = regex.Match( "192.168.123.1" ); string replace = @"${part4}.${part3}.${part2}.${part1}" + @" (the reverse of $&)"; Console.WriteLine( regex.Replace("192.168.123.1", replace) ); } }