output formatting - How to make rational expressions formatted using FractionBox

This is what I get using Mathematica 9.0.1.

enter image description here

Instead I want this:

enter image description here

I know how to use MakeBoxes, Format and I thought this code would work:

MakeBoxes[(1/2)(expr_), StandardForm]:=
FractionBox[ MakeBoxes[expr, StandardForm], MakeBoxes[2, StandardForm]]

But the code above and all variations that I tried have no effect. This has to be automated to get the look I want. My actual application is much more complicated.

Answer

There are multiple internal forms of x / 2. I ran into the same problem here (with 1/4):
Using Hold correctly with Simplify and ComplexityFunction

This appears to work in all cases:

MakeBoxes[expr_ / 2 | Rational[1, 2] expr_, fmt_] := 
  FractionBox[MakeBoxes[expr, fmt], "2"]

(3 + Sin[t])/2

Mathematica graphics

You can better see what is going on with FullForm:

HoldForm @ FullForm[expr_/2]
HoldForm @ FullForm[Rational[1, 2]*expr_]

Times[Pattern[expr,Blank[]],Power[2,-1]]

Times[Rational[1,2],Pattern[expr,Blank[]]]

You need to cover both the Power[2,-1] and Rational[1, 2] cases with your pattern.

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