This is the regular expression validation to accept numbers only. 

Generally I used to validate textboxes that has to take digits as inputs we can specify range too. 

The Regular expression is as follows

@"^\d{0,9}$" 

This Regular expression is tested and I would really welcome any corrections and suggestions. 

The usage is as follows.

private void button1_Click(object sender, EventArgs e)
{
    if (Regex.IsMatch(textBox1.Text, @"^\d{0,9}$"))
    {
        MessageBox.Show("ok");
    }
    else
    {
        MessageBox.Show("is not matched");
    }
} 

This is code snippet for validating textbox value for numbers between range 0 to 9.