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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 the Wolfram Language.

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$PreRead

$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 the Wolfram Language.

Details

  • $PreRead is always applied to each complete input string that will be fed to the Wolfram Language.
  • In multiline input with a text‐based interface, whenever each line is terminated, $PreRead is typically applied to the input so far.
  • $PreRead is applied to all strings returned by a $SyntaxHandler function.
  • $PreRead is applied before InString[n] is assigned.

Examples

Basic Examples  (3)

By default, $PreRead is not defined, meaning input strings are sent directly to evaluation:

Wolfram Language code: $PreRead

Use "hf" as an alias for HoldForm:

Wolfram Language code: $PreRead = ReplaceAll[#, "hf" -> "HoldForm"]&
Wolfram Language code: {hf[2 + 3], hf[2 / 3]}
Wolfram Language code: FullForm[%]

Restore $PreRead to its default value:

Wolfram Language code: $PreRead=.

Have the Wolfram Language print the FullForm of parsed input expressions before evaluation:

Wolfram Language code: $PreRead = (Print[FullForm[#]];#)&;
Wolfram Language code: a + b^2
Wolfram Language code: a b(c + d)

Restore $PreRead to its default value:

Wolfram Language code: $PreRead=.
Wolfram Research (1991), $PreRead, Wolfram Language function, https://reference.wolfram.com/language/ref/$PreRead.html.

Text

Wolfram Research (1991), $PreRead, Wolfram Language function, https://reference.wolfram.com/language/ref/$PreRead.html.

CMS

Wolfram Language. 1991. "$PreRead." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/$PreRead.html.

APA

Wolfram Language. (1991). $PreRead. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/$PreRead.html

BibTeX

@misc{reference.wolfram_2026_$preread, author="Wolfram Research", title="{$PreRead}", year="1991", howpublished="\url{https://reference.wolfram.com/language/ref/$PreRead.html}", note=[Accessed: 13-June-2026]}

BibLaTeX

@online{reference.wolfram_2026_$preread, organization={Wolfram Research}, title={$PreRead}, year={1991}, url={https://reference.wolfram.com/language/ref/$PreRead.html}, note=[Accessed: 13-June-2026]}