One may observe that MakeBoxes does not give the actual Box form of various expressions:

MakeBoxes[{1*^4, 000123, a*b c}] 

RowBox[{"{", RowBox[{"10000", ",", "123", ",", RowBox[{"a", " ", "b", " ", "c"}]}], "}"}] 

The 1*^4 was expand, the leading zeros of 000123 were stripped, and the distinction between * and (space) was lost.

Also, MakeBoxes doesn't like syntactically invalid or incomplete strings:

MakeBoxes[0001+`1,*^6*a b] 

Syntax::sntxi: Incomplete expression; more input is needed.

How can one get the actual Box form visible with Show Expression (Ctrl+Shift+E) without copy & paste?

One may observe that MakeBoxes does not give the actual Box form of various expressions:

MakeBoxes[{1*^4, 000123, a*b c}] 

RowBox[{"{", RowBox[{"10000", ",", "123", ",", RowBox[{"a", " ", "b", " ", "c"}]}], "}"}] 

The 1*^4 was expanded, the leading zeros of 000123 were stripped, and the distinction between * and (space) was lost.

Also, MakeBoxes doesn't like syntactically invalid or incomplete strings:

MakeBoxes[0001+`1,*^6*a b] 

Syntax::sntxi: Incomplete expression; more input is needed.

How can one get the actual Box form visible with Show Expression (Ctrl+Shift+E) without copy & paste?