how to count vowel given string (in textbox) using asp.net ?

Dim pattern As String = "[aeiou]"

TxtCountResult.Text = System.Text.RegularExpressions.Regex.Matches(TxtInputString.Text, pattern).Count

 string str = TextBox1.Text;

            Label1.Text = Convert.ToString(Regex.Matches("str", "aeiouAEIOU").Count);

            Response.Write(Label1.Text);

string pattern = "[aeiou]";

        string str = TextBox1.Text;

        Regex reg = new Regex(pattern, RegexOptions.IgnoreCase | RegexOptions.ExplicitCapture);

        Label1.Text = Convert.ToString(reg.Matches(str).Count);