Thanks Carl. That has one minor drawback. It pastes the expression in literally and not as a syntactic unit. Of course, the user could always type a set of parentheses before pasting but might often forget. If it was pasted after a 'z' in an existing Input Cell, the 'z' would only be associated with the first term in the expression. So I went to a box structure as follows.

expr = N[x/3 + 2 y/3]
PasteButton["1/3", 
 RawBoxes@StyleBox[RowBox[{"(", ToBoxes[expr], ")"}], 
   NumberMarks -> False]]

When this is pasted into an Input Cell that contains an initial 'z' we now obtain:

z (0.333333 x + 0.666667 y)

This maintains full machine precision in the notebook, but not in the posting. It is now also editable.

Yes! Both of those solutions worked. An ExpressionCell in a longer expression seems to be something like an InlineCell in a Text Cell. In my StyleSheet I use a slight color on Input Cells and no color on Output Cells. When Neil's Input Cell is evaluated I obtain an Output Cell but the ExpressionCell has the Input coloring. Michael's formulation solves that minor problem.

A nice collaboration between two experts! Many thanks.

David,

Can you initially create the Input cell into which the code is pasted with the Cell option NumberMarks -> False? Or can you set the option after the pasting?

Or maybe you can do something with ButtonBoxOptionsButtonFunction. See CurrentValue[{StyleDefinitions, "Paste"}] for a hint.

David,

You can slightly modify Michael's solution to do what you want including adding to the expression.

PasteButton["1/3", 
 ExpressionCell[expr, "Input", NumberMarks -> False]]

Regards

oops -- I just realized that this loses the extra precision that you wanted...

Thanks Neil. That does paste into an Input Cell with the desired print precision but unfortunately I was deficient in my example. Suppose we have an expression with 2D Boxes such as:

expr = N[x^2/3 + 2 y^2/3]

Then we lose the regular StandardForm.

(Also the real numbers would come from a longer calculation and we couldn't just revert back to the rational expression.)

David,

Use

PasteButton["1/3", expr // OutputForm]

This will do what you want. It still has the full resolution -- evaluate the result and you can check that all the digits are still there by doing //FullForm.

Regards,

Neil