This is documentation for Mathematica 3, which was
based on an earlier version of the Wolfram Language.
 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]=