based on an earlier version of the Wolfram Language.
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.
x_ — any expression, named
x__ — sequence of expressions
a|b|c — alternative forms
Cases — elements of a list or expression matching a given pattern