Wolfram Language & System 10.3 (2015)|Legacy Documentation
One of the unique strengths of the Wolfram Language is its powerful and succinct—yet highly readable—symbolic pattern language. Convenient both for immediate use in individual functions, and for systematic large-scale programming, the Wolfram Language's pattern language generalizes concepts like regular expressions to describe general patterns for arbitrary symbolic structures.
x_ — any expression, to be referred to as x
Except — anything except a specified pattern
OrderlessPatternSequence — elements in any order
Default — predefined default arguments for a function
StringExpression — symbolic string pattern