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.
| Pattern | Meaning |
_ |
any expression |
_h |
any expression with head h |
__ |
any sequence of one or more expressions |
__ |
any sequence of zero or more expressions |
x_ |
named pattern |
_:value |
optional pattern |
Except[p] |
any expression except one that matches p |