This is documentation for Mathematica 8, which was
based on an earlier version of the Wolfram Language.
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The Representation of Textual Forms

Like everything else in Mathematica the textual forms of expressions can themselves be represented as expressions. Textual forms that consist of one-dimensional sequences of characters can be represented directly as ordinary Mathematica strings. Textual forms that involve subscripts, superscripts and other two-dimensional constructs, however, can be represented by nested collections of two-dimensional boxes.
One-dimensional stringsInputForm, FullForm, etc.
Two-dimensional boxesStandardForm, TraditionalForm, etc.

Typical representations of textual forms.

This generates the string corresponding to the textual representation of the expression in InputForm.
Click for copyable input
FullForm shows the string explicitly.
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Here are the individual characters in the string.
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Here is the box structure corresponding to the expression in StandardForm.
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Here is the InputForm of the box structure. In this form the structure is effectively represented by an ordinary string.
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If you use the notebook front end for Mathematica, then you can see the expression that corresponds to the textual form of each cell by using the Show Expression menu item.
Here is a cell containing an expression in StandardForm.
Here is the underlying representation of that expression in terms of boxes, displayed using the Show Expression menu item.
ToString[expr,form]create a string representing the specified textual form of expr
ToBoxes[expr,form]create a box structure representing the specified textual form of expr

Creating strings and boxes from expressions.