The program creates a Regex object, passing its constructor a regular expression pattern that will identify text to replace. It calls the object's Replace method, passing it the replacement pattern.

private void btnGo_Click(object sender, EventArgs e)
{
    Regex reg_exp = new Regex(txtPattern.Text);
    lblResult.Text = reg_exp.Replace(
        txtTestString.Text,
        txtReplacementPattern.Text);
}

In this example, the search pattern is "[aeiouAEIOU]" and the replacement pattern is "." so the program replaces all instances of vowels with periods.