string str = "1234456789         123     456";
            Regex reg = new Regex(@"\s+",RegexOptions.IgnorePatternWhitespace);
            str=reg.Replace(str, " ");

------解决方案--------------------

C# code
        string filename = @"c:\test.txt";
        string s = File.ReadAllText(filename);
        string r = Regex.Replace(s, @"[\t ]+", "  ");
        File.WriteAllText(filename, r);

------解决方案--------------------
C# code
string str="1234456789";
str=str.Replace("5","5 ");