$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.

DetailsDetails

  • $PreRead is always applied to each complete input string that will be fed to the Wolfram Language.
  • In multiline input with a textbased 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.

ExamplesExamplesopen allclose all

Basic Examples  (3)Basic Examples  (3)

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

In[1]:=
Click for copyable input
Out[1]=

Use as an alias for HoldForm:

In[1]:=
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Out[1]=
In[2]:=
Click for copyable input
Out[2]=
In[3]:=
Click for copyable input
Out[3]//FullForm=

Restore $PreRead to its default value:

In[4]:=
Click for copyable input

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

In[1]:=
Click for copyable input
In[2]:=
Click for copyable input
Out[2]=
In[3]:=
Click for copyable input
Out[3]=

Restore $PreRead to its default value:

Introduced in 1991
(2.0)