This is documentation for Mathematica 3, which was
based on an earlier version of the Wolfram Language.
View current documentation (Version 11.2)
 Documentation / Mathematica / Built-in Functions / Programming / Rule Application  /
ReplaceAll

  • expr /. rules applies a rule or list of rules in an attempt to transform each subpart of an expression expr.
  • Example: x + 2 /. x -> a.
  • ReplaceAll looks at each part of expr, tries all the rules on it, and then goes on to the next part of expr. The first rule that applies to a particular part is used; no further rules are tried on that part, or on any of its subparts.
  • ReplaceAll applies a particular rule only once to an expression.
  • Example: x /. x -> x + 1.
  • See the notes on Replace for a description of how rules are applied to each part of expr.
  • expr /. rules returns expr if none of the rules apply.
  • See the Mathematica book: Section 2.4.1.
  • See also: Rule, Set, MapAll.

    Further Examples

    ReplaceAll replaces once at all levels of the expression.

    In[1]:=

    Out[1]=

    On the other hand, Replace applies only to the whole expression.

    In[2]:=

    Out[2]=

    In[3]:=

    Out[3]=

    ReplaceAll applies the set of rules exactly once on each part of the expression.

    In[4]:=

    Out[4]=

    ReplaceRepeated uses ReplaceAll repeatedly until the expression stops changing.

    In[5]:=

    Out[5]=