The Mathematica pattern language is a rich system for matching and replacing parts of Mathematica expressions. A full description of the pattern language can be found in the main Mathematica documentation, http://reference.wolfram.com/mathematica/guide/RulesAndPatterns.html.
A quick summary of pattern primitives is shown in the following table.
|any expression with head h
|any sequence of one or more expressions
|any sequence of zero or more expressions
|any expression except one that matches p