Then don't do
richTextBox1.AppendText
Put it into different text box instead.
I would suggest using a string builder:
StringBuilder sb = new StringBuilder();
string spacer = "";
for (int i = 0; i < mc.Count; i++)
{
sb.Append(string.Format("{0}Match[{1}]: {2}",spacer, i, mc[i].Value));
spacer = " ";
}
myTextBox.Text = sb.ToString();
Note that I moved the init of spacer outside your loop and changed it inside, and deleted the redundant GroupCollection - since it can't be accessed outside the loop, it is irrelevant.