how to validatate (restrict string )allow number followed by upto four decimal by regular expression in c# web application

try this

If you want the user to be able to type 14 digits without any decimals or up to 11 digits with two decimals (and not allow just a single decimal digit), you could do this:

^(?:\d{1,14}|\d{1,11}\.\d\d)$

if your want decimal upto four places

^(?:\d{1,14}|\d{1,11}\.\d\d\d\d)$

you know what I dont really have that experience in regular expression but here is an answer that might help you

put this before the event

static int afterdot=0;
static bool dot=false;

at text changed event put this code

for(int i=0;i<textbox1.text;i++)>
{
   if(int.TryParse(textBox1.Text[i],int x))
   {
      if(dot)
      {
         afterdot++;
         if(afterd>4)
         {
            textBox1.Text=textBox1.Text.Remove(i);
         }
      } 
   }
   else
   {
      if(textBox1.Text[i]=="." && dot==false)
      {
         dot=true;
      }
      else
      {
         textBox1.Text=textBox1.Text.Remove(i);
      }
   }
}

I hope I helped
:-)

toniyo jackson regular expression is working

u forgot to put @ before expression

u can do like this

string id = "123.2345";
            string idexpr=@"^\d{1,3}(\.\d{0,4})?$";
            if(!Regex.IsMatch(id,idexpr))
            {
                MessageBox.Show("Invalid");
            }

But, sincerely, why code it? Why use complicated Regexes? Why disregard the wonderful work done by the Forms team? Why not use:
1. A NumericUpDown with Decimals = 4;?
2. A MaskedTextBox with Mask = "read the documentation / I don't use this Control often";?

<asp:TextBox ID="txtNo" Width="70px" runat="server"
MaxLength="7" Height="16px"></asp:TextBox>;
<cc1:FilteredTextBoxExtender ID="FilteredTextBoxExtender4" runat="server" FilterType="Numbers" TargetControlID="txtNo">
</cc1:FilteredTextBoxExtender>;