This is documentation for Mathematica 6, which was
based on an earlier version of the Wolfram Language.
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Rules & Patterns
At the core of Mathematica's symbolic programming paradigm is the concept of transformation rules for arbitrary symbolic patterns. Mathematica's pattern language conveniently describes a very general set of classes of expressions, making possible uniquely readable, elegant and efficient programs.
lhsrhs (Rule) representation of replacement rule
expr/.rules (ReplaceAll) apply replacement rules to an expression
x_ any expression, named x
x__ sequence of expressions
a|b|c alternative forms
Repeated(..)  ▪ Except  ▪ Longest  ▪ Shortest  ▪ ...
Cases elements of a list or expression matching a given pattern
Position  ▪ MemberQ  ▪ MatchQ  ▪ Count  ▪ DeleteCases  ▪ ...
lhs=rhs, lhs:=rhs make assignments for arbitrary patterns