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Regex Replace

using System; using System.Text.RegularExpressions; using System.Reflection;   class Class1   {     [STAThread]     static void Main(string[] args)     {             Regex cardExp = new Regex( @"(\d{4})-(\d{4})-(\d{4})-(\d{4})" );                          string safeOutputExp = "$1-xxxx-xxxx-$4";             string cardNum;             cardNum = "asdf-asdf-asdf-asdf";                      Console.WriteLine( cardExp.IsMatch( cardNum ) );             Console.WriteLine( "Secure Output Result = {0}", cardExp.Replace( cardNum, safeOutputExp ));             TestManualGrouping();             CompileRegex( cardExp.ToString(), @"regexlib" );     }                  static void CompileRegex( string exp, string assemblyName )         {             RegexCompilationInfo compInfo = new RegexCompilationInfo( exp, 0, "CreditCardExp", "", true );             AssemblyName assembly = new AssemblyName();             assembly.Name = assemblyName;             RegexCompilationInfo[] rciArray = { compInfo };             Regex.CompileToAssembly( rciArray, assembly );         }         static void TestManualGrouping()         {             Regex cardExp = new Regex( @"\d{4}" );             string cardNum;             cardNum = "asdf-asdf-asdf-asdf";             if( cardExp.Matches( cardNum ).Count < 4 )             {                 Console.WriteLine( "Invalid card number" );                 return;             }             foreach( Match field in cardExp.Matches( cardNum ))             {                 if( field.Success == false )                 {                     Console.WriteLine( "Invalid card number" );                     return;                 }                 if( field.Index == 5 || field.Index == 10 )                 {                     Console.WriteLine("-xxxx-");                 }                 else                 {                     Console.WriteLine(field.Value);                 }             }         }   }