Anyone know how to use regex to duplicate a specific character in a string?

I have a path that is entered like this:

C:/Example/example

I would like to use regex (or any other method) to display it like this:

C://Example//example

Is it possible?

This is where I'm getting the file path

private void btnSearchImage_Click_1(object sender, EventArgs e)
    {
        OpenFileDialog ofd = new OpenFileDialog();

        ofd.Filter = "Image Files(*.jpg; *.jpeg; *.gif; *.bmp)|*.jpg; *.jpeg; *.gif; *.bmp"; 

        if (ofd.ShowDialog() == System.Windows.Forms.DialogResult.OK)
        {
            string filenName = ofd.FileName;
            pictureBox1.Image = new Bitmap(filenName);
            string path = filenName;
            txtimgPath.Text = path;

        }


    }

Thanks