How to write requler expression to my condtion

i have one textbox and one requler expression and i need to write the requler expression to textbox..and textbox maximum lenght is 15 characters my condition is
1.user is enter digits up to 15 digits -- 12345 .......(with out decimal)
       (or)
2. he may be enter like this -- 123456.8 (with decimal with 1 digit)
          (or)
3. like this --- 12345678.56 (decimal with 2 digits)
But always after decimal enter only 2 digits not morethan to allowed can give the expression to my contion..always i need to caliculate with textlenght with decimal (ex:- 123.45  lenght is --> 6)

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