For Child Controls in the Gridview we don't have the direct events
so first we have to get the TextBox value like this
TextBox Foottxtboxunitrate = (TextBox)ProjectOnFormGrid.FooterRow.FindControl("Foottxtboxunitrate");
then
Regex amountpattern = new Regex(@"^[0-9]*(\.)?[0-9]{0,1}?$");
if (amountpattern.IsMatch(Foottxtboxunitrate.Text.Trim()) == false)
{
lblError.Text = "Enter valid Rate amount with maximum 1 decimal places. ";
lblError.Visible = true;
Foottxtboxunitrate.Focus();
return;
}