i have a WPF TextBox that user can type number in that . now i am searching for a string format that can separate TextBox number each 3 point (like 0,0) but i want separate text with Slash or Back Slash or another character. and we do not know how much point our number has. i am searching for string format not Linq solution or etc . i read Microsoft help but cant find any way . sample = 123456789 == 123/456/789 (good) --- 123,456,789 (bad)