Rules & Patterns
At the core of the Wolfram Language's symbolic programming paradigm is the concept of transformation rules for arbitrary symbolic patterns. The Wolfram Language's pattern language conveniently describes a very general set of classes of expressions, making possible uniquely readable, elegant and efficient programs.
Rules »
lhsrhs (Rule) — representation of replacement rule
expr/.rules (ReplaceAll) — apply replacement rules to an expression
RuleDelayed(:>) ▪ ReplaceRepeated(//.) ▪ Condition(/;) ▪ ...
Patterns »
x_ — any expression, named 
x__ — sequence of expressions
a|b|c — alternative forms
Repeated(..) ▪ Except ▪ Longest ▪ Shortest ▪ ...
Pattern Matching Functions »
Cases — elements of a list or expression matching a given pattern
Position ▪ MemberQ ▪ MatchQ ▪ Count ▪ DeleteCases ▪ ...