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$Pre is a global variable whose value, if set, is applied to every input expression.
Unless $Pre is assigned to be a function which holds its arguments unevaluated, input expressions will be evaluated before $Pre is applied, so the effect of $Pre will be the same as $Post.
$Pre is applied to expressions, while $PreRead is applied to strings which have not yet been parsed into expressions.
See the Mathematica book: Section 2.13.1.
See also: $Post.