WOLFRAM

Wolfram Language & System Documentation Center
Wolfram Language Home Page »

$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)Summary of the most common use cases

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

In[1]:=1
✖
https://wolfram.com/xid/05gqagdgc6-kf29df
Out[1]=1

Use "hf" as an alias for HoldForm:

In[1]:=1
✖
https://wolfram.com/xid/05gqagdgc6-icz935
Out[1]=1
In[2]:=2
✖
https://wolfram.com/xid/05gqagdgc6-chdxpx
Out[2]=2
In[3]:=3
✖
https://wolfram.com/xid/05gqagdgc6-b1gwon

Restore $PreRead to its default value:

In[4]:=4
✖
https://wolfram.com/xid/05gqagdgc6-cilnr8

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

In[1]:=1
✖
https://wolfram.com/xid/05gqagdgc6-c62nty
In[2]:=2
✖
https://wolfram.com/xid/05gqagdgc6-hbwimy
Out[2]=2
In[3]:=3
✖
https://wolfram.com/xid/05gqagdgc6-gcaofv
Out[3]=3

Restore $PreRead to its default value:

In[4]:=4
✖
https://wolfram.com/xid/05gqagdgc6-hxfgib
Wolfram Research (1991), $PreRead, Wolfram Language function, https://reference.wolfram.com/language/ref/$PreRead.html.
✖
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.

✖
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.

✖
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

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

BibTeX

@misc{reference.wolfram_2025_$preread, author="Wolfram Research", title="{$PreRead}", year="1991", howpublished="\url{https://reference.wolfram.com/language/ref/$PreRead.html}", note=[Accessed: 26-March-2025 ]}

✖
@misc{reference.wolfram_2025_$preread, author="Wolfram Research", title="{$PreRead}", year="1991", howpublished="\url{https://reference.wolfram.com/language/ref/$PreRead.html}", note=[Accessed: 26-March-2025 ]}

BibLaTeX

@online{reference.wolfram_2025_$preread, organization={Wolfram Research}, title={$PreRead}, year={1991}, url={https://reference.wolfram.com/language/ref/$PreRead.html}, note=[Accessed: 26-March-2025 ]}

✖
@online{reference.wolfram_2025_$preread, organization={Wolfram Research}, title={$PreRead}, year={1991}, url={https://reference.wolfram.com/language/ref/$PreRead.html}, note=[Accessed: 26-March-2025 ]}