front end - On an existing notebook, how to transform all "double brackets" into "DoubleBracket"

Sometimes, having all double brackets [[...]] transformed into \\[LeftDoubleBracket] ... \\[RightDoubleBracket] can really facilitate code reading.

On a given notebook, how to transform all occurrences automatically (on input cells)?

The other way around is easier, but a couple of buttons interface will get extra... thanks.

Answer

Palette button

CreatePalette[
 Button["Compact Part",
   Do[
     With[{
       celldata = NotebookRead[cell],

       pattern = RuleDelayed[
         RowBox[{h__, "[", RowBox[{"[", spec___, "]"}], "]"}],
         RowBox[{h, "\[LeftDoubleBracket]", RowBox[{spec}], 
            "\[RightDoubleBracket]"}]
       ]
     },
     If[
       Not @ FreeQ[celldata, First @ pattern],
       SelectionMove[cell, Cell, All, AutoScroll -> False];
       NotebookWrite[

         InputNotebook[], 
         ReplaceRepeated[celldata, pattern], AutoScroll -> False
       ];
     ]
   ];
   ,
   {cell, Cells[InputNotebook[], CellStyle -> "Input"]}
   ];
   MessageDialog["Done"];


]]

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It may happen that it will miss an example from time to time, it should be rare though. In heavily edited notebook I have faced box structures that were not "proper" but good enough for FrontEnd to understand. So it may happen this "proper" pattern won't catch them.