How to remove space coming in blank tag

That's the same as me (.NET Framework 4.0) though mine's working fine.

Can you tell from Intellisense when you type Regex.Replace whether the overload which takes 4 parameters is acceptable?

http://msdn.microsoft.com/en-us/library/taz3ak2f(v=vs.100).aspx

I simply don't understand that error as the static Regex.Replace method does have an overload which takes three string parameters and AFAICR always has had:

http://msdn.microsoft.com/en-us/library/e7f5w83z(v=vs.100).aspx

What version of .NET are you using?

Try:

using System.Text.RegularExpressions;
using System.IO;

// ....

string text = File.ReadAllText("ashu.xml");
string regExp = @"(<\S+)\s(/>)";
text = Regex.Replace(text, regExp, "$1$2");
File.WriteAllText("ashu.xml", text);