This is documentation for Mathematica 5, which was
based on an earlier version of the Wolfram Language.
View current documentation (Version 11.1)

Documentation / Mathematica / The Mathematica Book / Principles of Mathematica / Patterns /

2.3.12 Verbatim Patterns

Verbatim patterns.

Here the x_ in the rule matches any expression.

In[1]:= {f[2], f[a], f[x_], f[y_]} /. f[x_] -> x^2


The Verbatim tells Mathematica that only the exact expression x_ should be matched.

In[2]:= {f[2], f[a], f[x_], f[y_]} /. f[Verbatim[x_]] -> x^2