Start by simplifying your regex:
<.+?>
Will catch just the bits you want.
Then, I'd do it like this:
private void myButton_Click(object sender, EventArgs e)
{
string source = @"Dear <customer>
This is to let you know that your following Tickets have been confirmed.
<Orders>
<OrderNo> <OrderType> <OrderAmount>
<Orders>
Total Amount is <Total>
Thanks for doing business with us.
Best Regards
Team";
string regExp = @"<.+?>";
Regex regx = new Regex(regExp);
string newData = regx.Replace(source, ReplaceData);
}
public string ReplaceData(Match m)
{
switch (m.Value)
{
default: return m.Value;
case "<Orders>": return "----ORDERS DETAIL----";
case "<OrderNo>": return "----ORDER NO----";
...
}
}
Try it and you'll see what I mean!