On the decimal field(MUST BE DECIMAL FIELD object), apply the below patterns.
1. Set the display pattern as : num{zzz,zzz,zz9.99}
2. dont put any patterns on Edit and Validation Tabs
3. Set the Data pattern as: num.decimal{}
Now put the Object properties of the decimal field as:
select the check boxes of Limit Leading digits and Limit Trailling digits and place the 9 and 2 respectively on the boxes(9 and 2 derived from ur display pattern of (num{zzz,zzz,zz9.99}), can be changed to ur needs.
With the above things in place, the decimal field cannot allow any characters other than the numaricals and dot(.)
Hope this helps.