$Pre

$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.

Further Examples