I tried this in button click try as per you requirement
Add this regex and make a try
[A-Za-z]\w*
string strText = textBox2.Text;
Regex reg = new Regex("^[A-Za-z]+\\d{0,9}$");
if (reg.IsMatch(textBox2.Text))
{
}
else
{
MessageBox.Show("Invalid");
}
or you can write some thing like this
if(textBox2.length>1)
{
txt = textBox2.substring(0,1);
if(!((txt >= 'a' && txt <= 'z') || (txt >= 'A' && txt <= 'Z')))
}
private void button1_Click(object sender, EventArgs e)
{
string strText = textBox2.Text;
Regex reg = new Regex("^[A-Za-z]+\\w*");
if (reg.IsMatch(textBox2.Text))
{
MessageBox.Show("Valid");
}
else
{
MessageBox.Show("Invalid");
}
}