asp.net webcontrols: How to insert comma to a number entered into a textbox?

Say if the number entered is 10000000000, when user press
enter key, the text in the textbox will be changed to
1,0000,000,000.

check the length and String.Insert the string and put it togather with comma
char
say calculate the length and start from right to left.
10000000000
use
str = str.Insert(1,",");
str is now 1,0000000000
str = str.Insert(6, ",");
str is now 1,0000,000000
Keep going,

Regards,

HD
Use custom length marker and say
[quoted text, click to view]