This design seems to be better, because you can use as many letters as you want to use as a pattern; moreover, you input is in one Textbox.
const string path = @"F:\TEMP\doctors.txt";
private void button1_Click(object sender, EventArgs e)
{
string input = File.ReadAllText(path);
string pattern = @"^" + textBox1.Text + @"\w*\r";
foreach (Match m in Regex.Matches(input, pattern, RegexOptions.Multiline))
{
MessageBox.Show(m.Value);
}
}