Need help in formating a string

Hi,
   In a form view,I am binding a value name1, and I need that value in this format ##-####-####-####. So i was trying like this:
string.Format("{0:##-####-####-####}",(string)Eval("name1"));
for example :
name1= 12345678912345;
output = 12-3456-7891-2345;
Can anyone help me.
Thanks in Advance,


                    



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