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$PreRead is a global variable whose value, if set, is applied to the text or box form of every input expression before it is fed to Mathematica.
$PreRead is always applied to each complete input string that will be fed to Mathematica.
In multiline input with a text-based interface, $PreRead is typically applied to the input so far whenever each line is terminated.
$PreRead is applied to all strings returned by a $SyntaxHandler function.
$PreRead is applied before InString[
] is assigned.
See the Mathematica book: Section 2.13.1.
See also: StringReplace, ToExpression.
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